I can’t seem to find a clear answer on the internet, or maybe I’m looking in the wrong places. From what I can gather a “key” is a collection of successive notes - how many is still unclear. Not sure if the likes of StevoMuso is still around, but does anyone know if a “key” is the same as, or comparable to, an octave? Also, what is the difference between, say, a G chord and the key of G … is the chord just all the notes in key G played simultaneously?
Rigil
For a piece of music, the key is a reference note around which the music centres. The so-called pitch of a musical note is defined by its frequency (vibrations per second, Hz) — the higher the frequency, the higher the pitch. There is a well-defined rule for changing notes successively up or down the key scale (including the semitones). If this rule is not followed exactly, the notes will be off key, either slightly higher (“sharp”, #) or lower (“flat”, b) in pitch, and the music will not sound quite right, at least to a trained ear.
An octave consists of seven major tones (C, D, E, F, G, A, B) plus five semitones (C#, Eb, F#, G#, Bb). Going up (or down) one full octave means doubling (or halving) the frequency of the notes, i.e. each C, D, … B of the next octave has twice (or half) the frequency of its counterpart in the present octave. A chord is a set of three or more notes that are in harmony and sound as if they were struck simultaneously (though they need not be exactly), and the dominant note gives the chord its name.
'Luthon64
Just to add, a G (major) chord is the 1st, 3rd and 5th notes of the G major scale played simultaneously and the chord will also be in the key of G major as it is composed of the notes of the G major scale.
In our western system of organising frequencies into “notes”, there are “whole notes” (white keys on piano) and “half notes” (black keys on piano). These are spaced in a certain pattern over the audible frequency spectrum. You’ll find eastern music sounds so different because they have alternate ways of arranging the “tones”, but I digress… If you play a bunch of the notes on a piano at the same time, or the wrong sequence in succession it can sound crappy. If you play in the key of “C”, you’re sticking mostly to the white notes only. However, say you want to start a scale (do re mi…) at the “D” note (reference note) and you play only the white keys, what results sounds incredibly wrong and is a result of the pattern we use to delimit our tones [I see mefi linked it]. So to make a scale starting at D sound good (or, fit our note system) we have to use some “black” notes at certain points to make it all fit and sound nice. We can play any tune, as long as we remember that instead of F, we have to play F# / F sharp / “a half tone” above F / the black note just above F, every time.
There are also “major” and “minor” keys, ie: There’s more than one way to arrange the black-notes resulting in a different “feel”. Generally a “minor” will feel a bit more “dark”, and a “major” a bit more “happy”. You can still play the same tune on either, but it will feel different on the ear.
So no, a key is not an octave (which is 8 successive “whole tones”: do re mi…). A key denotes which notes in an octave get played as sharps/flats to make it all sound in tune relative to some note, and so it repeats every octave you go up or down. Note how the notation is C#, not C# in octave X. All C’s get turned into sharps no matter the octave.
A chord is any number > 2 notes played at the same time. BUT, as described above, if the chord doesn’t adhere to the key, it will sound horrible. So if a guitar player plays a “chord” he’ll generally fret the correct notes on the neck, using 1/2 tones on the appropratie notes for that KEY, and strum all the strings (6 of them on a std guitar) at the same time. However he could also just strum the top 3 and it’s still a chord.
Thanks for the three quality answers.
So to sum up, am I right in saying that a key is not a collection of discreet notes within a frequency interval, while an octave is?
And a key is simply an instruction to play the sharps and flats instead of some major note?
(By the way, reason for this sudden interest in music theory is that I’ve pulled my 20 year old harmonica out of the mothballs and am trying to master Australia’s second national anthem. 
I see the instrument is in the key of G. When I blow or draw the corresponding notes on my son’s C-harmonica, they sound different, although Waltzing Matilda is exactly (to my ears anyway) playable in the same way on both. The C-harmonica just sounds sharper overall, hence my initial suspicion that “key” means “octave”.)
Rigil
Explaining the concept of a musical key is difficult to do without having a keyboard at hand and giving little musical illustrations as you go along.
If you look at a keyboard, you will see that there are 12 possible notes to choose from: 7 white notes and five black ones. As it turns out, in most pieces of western music, a piece will use a set of only 7 of those notes, or at least, the piece will be based on such a set of 7 of those 12 possible notes. The key tells you which seven. For example, a piece in C major will use only the white notes. A piece in G major will be the same except that instead of using the white note F, it uses the black note to the right of F, which is F sharp.
Put a different way, think of the solfege syllables (Do, Re, Mi, Fa etc.) as a very simple melody. If you start that melody on C, you use only the white notes until you are back at C an octave higher. If you start on G, and use only white notes, then the F will sound wrong. To keep the melody the same you’ll have to use F sharp instead of F. Thus it will be the same solfege song in two different keys.
A further complication here is that F sharp is the same note as G flat - there are some rules to decide whether you should talk about flats or sharps.
If, like me, you do not have perfect pitch, it will for the most part not matter what key a piece is in anyway. But with some instruments, e.g. guitar, it can be vastly easier to play in some keys than in others, thus it is often useful to transpose a piece written for one instrument into a different key when you want to play it on some other instrument.
Two centuries ago, the definitions of the pitches were different, and orchestras tended to play everything about a half tone lower than they do today. So to someone like Beethoven, who did have perfect pitch, modern performances of his works would all sound like they are in the wrong key. Assuming he could hear them, that is to say… 
Being not of a musical persuasion this all sound Greek to me but I, after a lot of hard work, can now play the “Sound of Silence” very well on any instrument.
Cool … light bulb moment achieved.
I have to assume that the musical know-how displayed above cannot be merely casual … so what instruments do you guys play or have tried to play? (IIRC Boogie was on classical guitar).
Rigil
You may also fare well with John Cage’s 4’33" 
Look it up on Wikipedia if this is also Greek to you.
Come to think of it, music terminology is Italian more than Greek.
I had a few months of piano lessons as a kid, until the teacher moved to another town.
In my early twenties I taught myself to play piano, using various books, but never achieved much in the way of virtuosity. I am way too lazy.
More recently I took up classical guitar, an instrument I love but on which I am something of a perpetual beginner. Lack of time for practice, and the above-mentioned laziness, quite effectively prevent me from becoming the next Segovia or Williams.
You may also fare well with John Cage's 4'33" :-)At what point are noises or scribbles art? Personally I prefer silence to rap and no picture to some "art".
Yeah I played piano when I was younger for a couple of years. Took a decade or so sabatical, then took up the classical guitar.
Upon reflection, I cannot allow that light bulb to keep on burning. As you may have guessed, there are plenty of complications.
For one thing, in just about any extended piece of classical music, the key tends to change, so a piece starting in, say, D major, may not stay in it for long before changing to some other key (the technical term for this is “modulation.”) However, up until the middle of the 19th century or so, pieces had one particular key on which they were based, and they would eventually return to it. This is perhaps partly what gives some of the music a sense of narrative, of a journey started and ended at the same place.
However, during the 19th century composers became ever more adventurous with this, and they ended up composing pieces which would end in a different key from the main one, or would contain chords or passages not clearly belonging to any particular key. Wagner was a major sinner in this regard, and he ended up looking tame compared to Debussy (whose music is absolutely magical, but decidedly an acquired taste - try out his “Prelude to the afternoon of a faun” or the piano piece “Reflections on the water” if you feel like pleasantly drifting off on a warm Sunday afternoon.)
In the early 20th century a bloke named Arnold Schoenberg began to see where this was going: keep on getting more adventurous with keys, and you’d end up with none at all. He decided to follow what he saw as a general trend to its logical conclusion and devised a system of composition in which the music would have no key at all. All 12 tones on the keyboard would carry equal weight, at least in theory, and such compositions became known as 12-tone works (which is more or less synonymous with a technique known as serialism, but never mind the finer distinctions.)
He and his disciples Anton Webern and Alban Berg (they became known as the “Second Viennese School”, the first being Mozart, Beethoven and Brahms) then created a body of music which, a century after its composition, still remains pretty startling and unmusical-sounding, at least to the casual listener. It’s notoriously free of any singable melodies, and many would argue that it has no redeeming features whatever. But it did give rise to some absolutely hilarious satire, of which this is my favourite:
There is of course another sense in which we use the word “key” in music, and that is the keyboard. Like this very old-fashioned one for example…
La máquina de escribir. L. Anderson. (Lio en los Grandes Almacenes)
Now that’s my type of music.
Alas, the writing is on the wall for it. Such concerts take more money and skill than modern society can come up with, and soon it will exist only on paper.