Sus chords
”Sus” stands for suspension and what happens in these chords are that the third (the second note) are being replaced with either a major second or a perfect four. When it is replaced with a major second the chord name is sus2, and when it is replaced with a perfect fourth the chord name is sus4. There are also extended suspended chords.
See diagrams of sus chords:
Theory
As mentioned above, there are several categories of suspended chords, but in most cases it is either a sus2 or a sus4.
Comparing the C major Chord with Csus2 and Csus4:
C: C – E – G 
Csus2: C – D – G 
Csus4: C – F – G
Formulas
The formulas are 1 - 2 - 5 (sus2) and 1 - 4 - 5 (sus4).
Other sus chords
Beside the common sus2 and sus4 chords there are more categories. A seventh or a ninth chord, for example, could be suspended.
Let us compare a C7 chord with C7 sus chords:
C7: C – E – G – Bb
C7sus2: C – D – G – Bb
C7sus4: C – F – G – Bb
Let us also compare a C9 chord with a C9sus4 chord:
C9: C – E – G – Bb – D
C9sus4: C – F – G – Bb – D
Among these extended suspended chords, the ninth suspended chord is the least common. C9sus4 is more commonly referred to as C11 (to be exact, it is an inverted C11 without a major third).
A less common category is the suspended 2/4, which include both the second and the fourth.
Let us compare regular suspended chords with a Csus2/4 chord:
Csus2: C – D – G
Csus4: C – F – G
Csus2/4: C – D – F – G
Alternative chord names
Besides the standard sus4/sus2 names, the annotation C4, D4, E4 and so forth can occur and often together with C3, D3, E3 and so forth (meaning a sequence from sus to major).
Chord categories
Major chords Minor chords Seventh chords Extended chords Sus chords Dim chords Aug Chords Add Chords Altered Chords
