Webmail accounts use POP3 , IMAP or something else? Why?

Depends on the provider.

Some ISP's will allow either one. Here are some differences between the two:

POP3 - (Post Office Protocol 3) is a "store-and-forward" type of system that delivers all of your email to you when you connect to the mail server. The emails are then stored on your computer after they are delivered.

IMAP - (Internet Message Access Protocol) keeps email on the server where it can be viewed. The message headers are downloaded to the client so that a list of available emails can be seen. Emails are not retrieved by the client unless they are asked for. Since a continuous connection is required for this type of system, it is typically used as internal company email systems where email is kept on a centralilzed server.

The POP3 system is most used by Internet providers since it is less expensive both in bandwidth and storage space.