There are basically two types of suspended; they are:
sus4 and sus2
The numbers beside the sus in the two types above refer to the numbers in the letters of music. Look at the illustration below:
C D E F G A B C
1 2 3 4 5 6 7 8
Now, 4 represents F, while 2 represents D.
So how do the suspended chords work?
The suspended replaces the major third in a chord with either 4 or 2.
Let us take the doh chord on the key of C for example
C E G
d m s
Now to make this chord a sus2 chord, we would identify the major third in it. The major third is the E note(look at the first illustration above). So the E note will be replaced with the D note to suspend the chord and make it a sus2 chord(remember D is 2 in the illustration above). so we would have
C D G
d r s
Likewise to make this same chord a sus3 chord, the major third which is E will be replaced with the fourth which is F(look at the illustration above to understand the third, second and fourths). So we would have
C F G
d f s
A chord without out a major third like C G is called an "open fifth".
So while you are playing suspended chords, it is necessary that you resolve back to a major chord.
A song that has the 1 4 5 1 progression, which is doh fah soh doh, can use suspended chords without having to move too far
doh fah soh doh
d m s d f s d r s d m s
C Csus4 Csus2 C