What do i need to know about a hamster before i get one?

i am getting a hamster soon,but i dont know everything about hamsters. i have had 3 gerbils i dont know if they are very alike,but i know there is some differance. please let me know all about hamsters. also should i get a dwarf hamster, a russian dwarf hamster or any other type of hamster that is good as a small pet . thanks.