Sup /adv/

I'm applying for grad school and I'm having problems coming up with answers to the essays they want me to write.

Some call them "Personal Statements", some call the "Educational and Career Goals", some call them "Letters of Interest", etc. Are these all the same document?

Does anyone have prior experience writing these sorts of essays? I'm an intelligent guy, applying for ecology/environmental biology positions but I've never really had to write essays about myself like this. I don't know what's relevant and what's not.