They are spam bots. They use a brute force type of hack where they just guess passwords and try to log on. They're not doing it to get any information from you or the site. Their sole purpose is to log in and make spam posts to the board. Usually selling porn, knock off sneakers, jewelry, viagra etc. They also add a bunch of links to their posts so their site gets a lot of 'back links' which looks good on the search engines. The bots also try registering their own accounts and post the same stuff. But thankfully ASF has enough of a registration process that the bots can't get through that way.
Like Heavy mentioned above, just make sure you have a decent password so they can't guess it, or at least make it very difficult.