Bulletin Board Service if I remember correctly. Typically
unsecured sites with no protection. Very often virus infected. Or
if you kenna Trojan before which attempts to take over your PC or
install software without your knowledge, those are the sites they
use as control location. For example, if the trojan includes a key
logger (i.e., records your every keystroke), that is where it will
send the information to.
Anyway, bottomline is try not to access any website which has bbs
in it. Also, if you do not know what MIRC is and you have this
program running on your PC, chances are you are already infected
with a trojan.
Just some PC knowledge I learn over the years. Anyway, if I am
wrong on any of the above, feel free to correct me. I am no expert
but a self-learn kind of guy.