[timvass]

Pretty much sort of yes.

If a song is in a certain key means it uses a particular group of notes called a scale. This scale starts on the note of the key you are in say C and goes up in steps (7 steps for C major) untill you are back to C again . Do ray me far so la tee Do if you like. If you play a group of notes together from this scale you get a chord .

So if I am playing in the key of C I will be using notes from the scale of C for the melody and groups of those notes for chords.Because C F ans G chords are made from those notes they will fit.

7ths minors and the rest are about adding extra notes to a chord .with out getting bogged down its a bit like mixing colours .

If you add a touch of white to blue you get light blue,do the same to green you get light green.They are both still the original colour green and blue but they have both been altered in the same way lightened .You may call the tinted colours pastel blue and pastel green,because thet have both been changed in the same way.

SO...... add a note to A and you get A7th still an A but different .
Make the same kind of change to a D chord and you get D7 still a D but coloured.

I once heard the note you add to make 7th called a "Tension note" which I think descibes its effect on the sound pretty well.

This a simplfication .There is a lot to learn but its good to have a idea of how it works