(I'm very surprised there was no node about this...)
ASClock is short for AfterStep Clock. asclock is, simply, a clock that works as a AfterStep wharfable application, or WindowMaker dockapp. Of course, it can be used independently in case you don't use the True Window Managers.
It is instantly recognisable. In it's default mode, it has a digital clock on the top, and a calendar (that looks like a wall calendar) under it.
Of course, many considered the clock somewhat boring. Thus, newer versions support themes...
ASClock also works as a GNOME panel applet, and most GNOME setups I've seen use it as its clock by default.