The suspended chord is another one of those chord types that attracts some confusion. In traditional theory, suspension is a term used when a chord tone is held over from a previously played chord and then resolved to the third of the new chord. That's the quick explanation, and one that we aren't going to go into detail because in modern music the suspended chord is considered a chord in it's own right.
For example, the C major triad consists of the notes C E and G which are the first, third and fifth scale degrees of the C major scale. The second scale degree is D and the fourth is F so if we replace the E with D we end up with a Csus2 and by replacing the E with F we get a Csus4 chord.
Suspended Chord formulas
There is some controversy surrounding the use of Sus2. This chord has evolved quite recently compared to the suspended fourth and while the theory behind it makes sense, there are some that argue it's usage because it's simply an inversion of a Sus4 chord. For an example, if you look at the notes in an Fsus2 chord you'll see that it's actually an inversion of a Csus4.
Whatever side of the fence you are on, you may as well get used to suspended seconds because it's very likely they're here to stay, even if they are a sus 4!