Im wondering what all the uses for the meta tag are???

i havent seemed to be able to locate this info...
Also, Can anyone fill me in as to how search engines such as yahoo find sites?
i mean, do i have to pay some sort of fee for them to add me in their list?
and if not, then how do i go about trying to make it so that my site shows up at a decent location on the search list?

C O N F U S E D