I do vinyasa yoga regularly, and I've done ashtanga a few times. If I'm not mistaken, ashtanga is just about progressing through different "series" (sun salutations, then the "primary" series; if it's an advanced class, then maybe some back bends, maybe some crazy legs-over-the-head stuff, maybe some other insane flexible-people shit). You also do a bunch of different locks (think feet behind the head). Ashtanga usually means "really fucking difficult" if you are neither flexible nor used to yoga. Of course, that's just my experience with it, and I don't know what all the names mean exactly either. I've done stuff like this before (see the picture below) and it practically broke me for about four days afterward.
biiiiig picture (Click to show)
It's also funny being home for break and being the youngest person at this studio I found nearby. All these young to middle-aged moms come in talking about their kids, and then once the class begins they just kick my ass doing standing splits and other crazy stuff.