What is POP3?
Answer IsPOP stands for Post Office Protocol. It is the protocol (a common language that is used by computers to exchange information between different hardware components, different computers over a network etc'. A computer protocol is pretty much the same as a regular human language) used to check any non-web-based Email account.
A web-based Email account is an Email box that can only be used through a certain website. During your stay in this website, you are exposed to ads which the company who provides you with this mailbox profits from. Web-based Email services are always free because instead of paying, you pay the company who handles your mailbox by watching these ads (unless this mailbox features some extremely special services or features that cost more money).
Hence, a non-web-based Email account is an account that is accessed using an Email client or a regular telnet client, if you know the necessary protocols (after you finish reading this tutorial and the Sendmail tutorial, you would know how Email works and how to compose and receive Email without the need of any program besides a telnet client).
Non-web-based Email accounts use two protocols - SMTP (Simple Mail Transfer Protocol) to send Email and POP (Post Office Protocol) to receive Email. You can study the SMTP protocol in the Sendmail tutorial, and as for POP - you'll get to learn it right now. In case you're wondering, POP3 is just a newer version of POP.
