Sus4 chords are formed with the second note in the chord raised one step. In addition to sus4, there is sus2. In the case nothing more than "sus" is mention, like for example Dsus, you can count on that sus4 are intended. See also common sus chords.
The sus4 chord is very close to the original major chord because only one note is changing. Let's compare C major and Csus4:
C major Csus4
There are two differences: 1) on the fourth string the tone is raised one semi-step from E to F, 2) you don't play the highest open string since this is an E note. When shifting from C to Csus4 you should therefore only add the little finger (4) and don't move the rest of the fingers.
A common practice is to alter the major with a sus chord, like D - Dsus4 - D.
In the Tom Petty tune "Free Fallin'" a certain chord progression is used in a big part of the song:
D - Dsus4 - D - Asus4.
Another progression with sus4 chords is this one: E - Esus4 - E - D - Dsus4 - D - A - Asus4 - A - E.