Servers make no sense... help!!! :'(

[Perdition]

i dont get servers. theres two type right? a main geographical server ie china, NA or EU? and then sub servers that are just a division of thye main ones. 
so how do  i know what main server is best for me and can i even change it if i wanted to???? and chan you change your sub to a server with less or more people? or are both of these fixed???
if someone can just explain to me. how i find out what server will be best and then how i change to that one. if that can be done? thanks all!