There are three or four versions of Android that are official: Standard Android, Android + SenseUI (on some HTC phones), Android + Rachel (on the Sony Experia X10), Android + MotoBlur on some Motorola phones (not the droid). You don't have any of those unless you fall into the category below...
There are also some custom ROMs around, where someone has taken Google's open source code for Android, and added their own stuff, or mixed it with stuff from later versions. An example of this is CyanogenMOD, by Cyanogen. You would know if you had a custom ROM, because you would have gone through the procedure to root your phone (get super user access), downloaded a custom ROM, and flashed it onto your phone. This is not the kind of thing one would forget doing, as for most people it's scary and you have to follow pages of instructions.