What kind of personal or private information from your computer are you giving a nosy website access to when you log in?

I'm apart of this super secret cloak and dagger type website where it seems everyone on the site is trying to figure out everyone else personal information (especially the site administrators.

I was wondering what kind of information is transferred from my computer (I.P. addresses, Browsing History, Data Cookies, File Sharing, My ISP, Pictures and documents) when I log in?

and how can I protect my private identity?