Basically, a 'five' chord is made up the root note: this is the first note of the scale which gives its name to the chord, then you add the fifth note of that scale. Eg C5 is made up of the notes C and G (C d e f G a b C), D5 is D and A (D e f# g A b c# D).

B5 isn't in the key of C major, as the 5 note of that chord is F# (B c# d# e F# g# a# B).

Sorry if the above is confusing, but I think, if you really want to get into understanding the theory behind keys, chords and such, then you could start with scales (keep this simple to begin with, such a Major, minor, and Pentatonic; then how chords are structured from the scales, starting with Triads).

As for rock songs, you will often find chords used as 'fillers' that do not fit in whatever key; or even find a complete change in key for some sections (which can also be found in songs of various genres).