chords are sets of 3-5 notes that all together resonate in one specific note. (barre chords/power chords, employ just the "bottom" and the "top" notes in a chord, and are very easy to use which is why most punk bands employ just barre chords.)
you can have 5 notes that when played together form a C, or a C# (sharp)
most rock music has three to four chords as the main basis for the song.
let's say a song starts in G, the guitarist strums the G chord 4 times, then he switches to a C chord. your ears can tell the difference, and the "change" is how he gets to that new chord.
real simple songs just go from chord to chord, a la ramones, and the most basic rock songs employ TWO chords only. trhink of some of the more basic misfits tunes.
more complex songs have a "bridge" between chords, a sequence of notes that gets you to the new chord, creating a nice emotional change in the listener. a great bridge can serve as the HOOK in a song or it can lead into the chorus hook.
it sounds more complex than it is, but it is just terminology
help me out people!