Newbie to WordPress – easy question about templates

Hello,
I am new to WordPress but I am not new to web design. I click on Appearance then Editor. On the right right I see 16 templates (footer, header, content, side, footer 2, etc). How can I tell if these are all used in my site are just leftover from the previous programmer?

At the bottom right, there is style.css and xstyle.css. Can two stylesheets be used? How can i tell if they are being used by my site?
Thanks!
Linda