A query is an object that requests information from a database and creates a dataset of the requested information. Baslicly one query means that PHP had to communicate with the database one time. There are no limits to queries, but the more queries you have, the longer it will take to load the page. This of course all depends on your server and hosting account. You run over 100 queries while installing vBulletin, so you won't have to worry about that to much.