IMAP and POP3 are protocols used to access and download email from email servers to your computer.
POP3 Mail
When using the POP3 protocol for accessing your email, your mail client (for example, Microsoft Outlook software on the Desktop PC) will download each mail from the server to your computer. The POP3 mothod removes your mail from the server when it is downloaded to your computer.
IMAP Mail
The mail remains on the remote server when accessing email using the IMAP mail protocol. This is similar to accessing a Gmail, or Yahoo mail account. With IMAP, you can view the mail stored on the server from your desktop computer, your phone, and webmail. If you delete mail when using your phone, it will automatically be deleted the next time you access your mail on your computer or webmail.
IMAP is a great choice for people who need to access their mail from multiple devices and is more commonly used than pop3 mail.
POP3, on the other hand, is great if you store a large amount of emails with large attachments. By downloading your email to your computer, you remove it from the server and avoid filling up your server space with old emails. POP3 method was more commonly used when the server space offered by mail providers was more limited than today. Downloading all mail (removing it from the server), would free up space on your mail account. The downside of downloading all of your mail to your computer is that you are responsible for backing this information on your computer.