| In general, it's a dominant 7th chord, in which the 4th replaces the 3rd. Gsus, for example, could be voiced G C D F. Reducing that to a triad would just give you G C D.
This comes, I believe, from what, in "Classical music" is referred to as a 4-3 suspension, whereby a dissonance is created, by replacing the 3rd with the 4th, and then the 4th resolving down a semitone to the 3rd. If you've got a keyboard handy, play a simple Gsus triad (G C D) followed by a G triad (G B D), and you'll hear what I mean.
However, in Jazz and popular music it's very common to simply play a sus chord, without resolving it (i.e. without the 4th resolving to the 3rd). Usually, sus chords are dominant in function. One common sus chord voicing, in both Jazz and Pop, is that of a major triad a tone below the root. For example, Gsus can be voiced as an F triad with a G bass. Technically, this could be referred to as G9sus, since it has the 7th (F) and 9th (A) in it. The important point is that it has a 4th instead of a 3rd.
There are of course, no rules about this, however. You can voice a sus chord with both a 4th and a 3rd. But, much more often that not, the 4th replaces the 3rd.
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