rush standards and pledging process

Can anyone offer insight into the type of fellows each fraternity usually gravitates towards giving bids to? I know every fraternity "only looks for the best," but in reality they each have certain aspects they look for (jocks, rich kids, some just let anyone in, etc...)

So, from your general knowledge, what sort of guys do each go for? And after that: what are the pledging processes like? Are any more difficult than others?