Difference Between SMTP and POP3 mail server

Somtimes we may get confused with mailserver operation.Here is the scrap to get over-it.
Basically there are two types of mailsever SMTP and POP3


SMTP: This server is called as MTA-Mail Transfer Agent.As the name indicates it transfers mail between two domains.In linux there two important SMTP servers -Sendmail and Postfix

POP3:This server is act as secondary to SMTP sever , that is it collects the mail from SMTP server and stores with in it.We can use any POP3 clients such as Thunderbird or Mozila to get mail from POP3 server and read the mail