Back in the day it was X-Pac, Road-Dogg and Triple H (maybe others, but those were the main members). I think Stephenie McMahon joined too with Triple H at some point.
Edit - The original DX was Shawn Michaels, Triple H, Chyna and Rick Rude. After Shawn Michaels left the company because of his back injury and Rick Rude leaving to WCW, Triple H controlled DX and re-constructed the team. The next formation was Triple H, X-Pac, The New Age Outlaws (Billy Gunn and Road Dogg) with Kane being an associated member. The next formation was Triple H, X-Pac, Billy Gunn, Road Dogg and Tori along side with Stephanie. Later on, Billy Gunn was dropped from DX and the team paired with Vince and Shane McMahon for awhile. Those were all the previous members.