Have a read on diatonic harmony.... it'll answer so many of your questions... it's a big topic and it will be easier to answer your questions when you have more of the information together.

It's all kind of inter-dependant, so when I try to explain something to you I'll end up using other terms you maybe haven't heard, which doesn't help you as much...

Start with this:

G Major Diatonic Harmony

I - G Major Chord / G Ionian Scale
II - A Minor Chord / A Dorian Scale
III - B Minor Chord / B Phrygian Scale
IV - C Major Chord / C Lydian Scale
V - D Major Chord / D Mixolydian Scale
VI - E Minor Chord / E Aeolian Scale
VII - F# Diminished Chord / F# Locrian Scale

The roman numerals are the 'scale degrees'. With seventh chords instead of normal triads, the order is the same as above, but the fifth degree (V) would be a D7 (dominant) chord rather than a major as it is above.

From the above, if someone you're jamming with says 'let's play a one four five in G', they want to jam on a G, C, D chord progression.

I hope that helps... :D