Since this question comes up all the time...
What is the difference between H.264, AVC, and x264?
It's a rather big difference, but not one that matters to the leechers, mostly.
H.264 and AVC are two different names of the ISO standard also known as MPEG4 Part 10 (ISO/IEC 14496-10). x264 is an IMPLEMENTATION of that standard.
To clarify, XviD is the name of an implementation of the ISO standard MPEG4 Part 2 (ISO/IEC 14496-2), also known as MPEG4 Visual, of which the Advanced Simple Profile is a part, while "ASP" or "MPEG4-2" is a name of the standard. The difference is that people tend to name their releases "XviD" if they were made with XviD, but for h264 stuff, "h264" is usually used no matter what encoder was used to create the file (of course, some people use "x264" anyway, increasing the confusion).
Hope that helped to clear out the confusions.
@LytHka: yes, higher quality IS possible. Compare gg's Ayakashi versions... same size, but the h264 version is better. Not that much, but it's definitely noticeable. XviD just doesn't handle sharp edges the way h264 does. Ayakashi is kind of a bad example, though, since it compresses pretty good anyway after I put it through dehaloing and antialiasing...