A suspended chord is a 'dissonant' sound which needs to be 'resolved'.

The commonest suspension in Classical music is the fourth, which resolves to the third -

C F G resolves to C E G.

Bach (for example) often added a suspended second -

C F G to C E G to C D G and back(h?) to C E G.

In reality you can change any note of a chord, but you will sometimes find it's not a 'sus' chord at all - if you move the G of a C triad up to A - C E G becomes C E A - you end up with the 'first inversion' of A minor... 'sus' chords are dissonant, not consonant.

I know I'm reiterating what was said before, but sometimes a different slant helps things (and sometimes it serves to fog!).