What are the differences between POP3 and IMAP?

"IMAP is fantastic if you are using the same email account on multiple devices, such as a desktop PC, mobile phone, laptop and web interface. Whenever you read an email on one device, that is synced to the IMAP server and then synced down to all the other devices and marked as read on every device.

POP3, on the other hand, downloads the emails to a client then (typically) deletes it from the server. So this means if you get an email whilst you are out and about on your mobile phone and read it, you will not get it on your desktop PC."

POP3 and IMAP are two different protocols used for viewing mail. The big difference is that POP3 downloads the messages to your client. IMAP leaves the email on the server while there is a basic cache stored in a desktop client. IMAP has the reputation of being used with webmail providers. IMAP has better support for multiple clients since the messages are stored on the server. More information can be found in those two links.

"POP3 (Post Office Protocol version 3) is a protocol used to retrieve email from the server. POP3 protocol was created because of the design of the electronic mail system that requires the server to accommodate email for a while until it’s taken by an eligible recipient.

IMAP (Internet Message Access Protocol) is a standard protocol for accessing / retrieving e-mail from the server. IMAP allows users to choose e-mail message that he will take, make a folder on the server, search for specific e-mail messages, even deleted e-mail messages on server. This ability is much better than POP (Post Office Protocol) which only allows to retrieve/ download all the messages without exception.

SMTP (Simple Mail Transfer Protocol) is a commonly used protocol for sending electronic mail on the Internet. This protocol is used to transmit data from computer sending email to the recipient mail server.

For reference, you could read this
http://signinx.com/difference-between-pop3-imap-smtp/"

"If you use e-mail from a provider, it would be best to consolidate it intoa Gmail account, to make sure you keep control over your own e-mail address. So many people that switch providers feel the pain of losing their e-mail address, and there's totally no need for that.

Just sign up for Gmail (or any other non-provider e-mail account that allows you to fetch with POP3), and follow the instructions onsite on how to enable Gmail to fetch POP3 from your provider.

Then, access Gmail (with your provider mail inside) using web, POP3 or IMAP as you see fit."

They're methods for retrieving email from a server. There are some nice comparisons, especially this one from UOregon. As others have said, the big difference is server-side versus client-side storage.