Customising authors page

Hi, I’ve read a few posts about something similar to this but haven’t quite understood it –

Where do I find the page that appears at this url – http://www.mydomain.com/author/authorname/ ?

Basically I want to customise the profile info that’s printed out on this page but I can’t find a page like author.php or similar.

Where is this page and what’s it called?

Any help would be great.

Thanx,
Mik.