If you're using email client software such as Outlook and Thunderbird you will notice these two protocols called POP and IMAP on the account setting. Before we begin let us find out what POP and IMAP stands for. POP is Post Office Protocol while IMAP is Internet Message Access Protocol.
IMAP is like a window to your email on the email server. If you check your email on your computer it retains the messages on the email server although you will see them appear on your computer. It also synchronizes the folders of your email client on your computer with that of the email server. POP on the other hand works the other way around. If you open your email client software on your computer it moves all the email from the email server to your computer. Although you will find an option on the email client softwares that you can retain a copy on server however by default that is disabled.
The advantage of IMAP over POP is you can access your email anywhere. You will find it useful if you are travelling because even while you are away from the office or school you still have an access to your email.