I want to know what is the actual different between them.
Anyone can tell me ??

User account = account a user has. In web hosting, it means a person is using web hosting services from a hosting company, so a user has an account with them.

POP3 account = Post Office Protocol 3 - a way of storing mail on a web server for someone to retrieve e-mail from.

Having a user account is like living in an apartment. It's yours with its rooms and its mail address.

Having a POP is like receiving your mail at your friend's apartment. You don't live there (not a user) but recieve email only.

Similarly, a user account provides you with privileges to use the web server and possibly the shell, in addition to email. A POP account is strictly an email address, no more and no less.