you might have seen the tags thing at the bottom of the page as you make a post.

if you fill in the tags it does several things,
firstly it index's everything in vbulletin and makes searching simple
secondly google and buddies love things like this, so you can drive traffic to your threads if you wish.
thirdly it gives us an idea about thread content, for example if you look at the serious guitar collection of van hielan you'll notice that the tag line is van halen.

have fun..