Right, but most of the time I see them it's as a Eadd9 or Esus2. The difference between those two chords is whether or not the 3rd exists, but they still always use the different terminology for the 2nd. I don't know why.

Well actually when it’s a Esus2, playing the 3rd is optional. Most of the times the reason you don’t play it on a guitar is because you don’t have enough strings. The same is true of an Esus4.

The reason for the change in terminology (“sus” vs” add”) is that by definition, chords are spelled in terms of thirds (1, 3, 5, 7, 9, 13, 15). A “standard” chord includes 1, 3, & 5, and then you start getting other notes in as you add the 7th. As I mentioned before, technically as you add in each note you also add in the thirds before it as well (so a 15th would have 1, 3, 5, 7, 9, & 13 as well), though they’re not always played. So “add” means to add onto the chord.

“Sus” means that it really isn’t part of the chord, but that it’s “suspended” from a previous chord. Or at least that’s how it started out. If you look back at Bach’s music you’ll see suspensions all over the place, but they’re never treated as part of the chord. They are held over from previous chords and resolved at some point into the new chord. A sus4 is generally resolved to the 3 that it is pulling toward. The sus2 has a less obvious resolution, since it is a whole step away from both the 1 and the 3.

As I said, suspensions were originally holdovers from previous chords, but in modern music we use these as simply part of the chords. If a guitarist or keyboardist is smart he or she will use this information to try and precede a “sus” chord with one that has the same note so that it really does feel like a suspension, but it’s no longer a real necessity.

So I hope that clears it up a bit. (And I think I’m pretty accurate about all of this, but it’s been awhile so I take no responsibility for any errors in my descriptions!)