rim and tire sizes

ok i want to know a couple things im having a hard time figuring out. how is the offset of a rim measured? would it be better to go with a 205 or 195,then which profile 195/50 or 205/50 or /45/? need some tips please? and if you could just provide a comparison for how i can figure out offset like +10 is____inches…

http://blog.tirerack.com/blog/hunters-ramblings/what-is-wheel-offset-and-backspace