POP and IMAP are protocols which allows you to download messages from mail servers onto your computer so you can access your mail with a program like Microsoft Outlook, Thunderbird, etc..., even when you aren't connected to the Internet.
What is Difference between IMAP and POP3 ?
POP3 is a protocol to download and locally store your e-mails. When your client connects it will download all mail stored in your mailbox and save it to your hard disk. It will then (by default) delete the copy that is held on our servers.
POP3 is good if you want to keep all mails you are sent as you are not limited by your mailbox quota once the mail has been downloaded or if you wish to keep local backups of your mail.
It is limited to only one connection (computer, phone or other device) at a time. In addition, once a message has been downloaded to a computer and removed from our server it can only be viewed on that computer.
IMAP4 is a protocol used to access and manipulate the contents of the mailbox on the server in real time. You can open mails, create folders to store mails, delete messages from the server and otherwise use the system as normal. The primary difference is that all messages are saved and stored on the server, not on your local computer
IMAP4 is good if you want multiple computers or devices (such as a smart phone) to be able to see the same mailbox and messages. It also allows you to access your mails via the Web-mail system when roaming.
It is limited in that your mailbox on the server has a maximum quota. You will need to manage the e-mails that you save and remove older ones or ones that are not required in order to ensure that the mailbox does not fill up and prevent you from receiving new messages.