IMAP and POP are the protocols which are used to allow you to download messages from mail servers on your computer and access them with the help of email mail clients (i.e. Microsoft Outlook, Mozilla Thunderbird etc.). The advantage to this technology is that you can access your emails via a feature-rich browser-independent mail client. In case of POP, you get offline access to old mails too.
IMAP and POP are two different protocols. There are many differences between these two. The main difference is that IMAP(Internet Messaged Access Protocol) always syncs with mail server so that any changes you make in your mail client (Microsoft Outlook, Thunderbird) will instantly appear on your webmail inbox.
On the other hand, in POP(Post Office Protocol), your mail client account and mail server are not synced. It means whatever changes you make to your email account in the mail client will not be transferred to the webmail inbox.
In simple terms, if you are using IMAP and mark a mail as read, it gets marked as read in your web based inbox too (because the changes are happening on the server). However, this won’t be the case if you are using POP, because the mails are downloaded to your PC and the changes won’t reflect on the server.
Different mail services have different settings for dealing with protocols. In Gmail you can find options to activate both the protocols: POP and IMAP (Go to settings -> Forwarding and POP/IMAP). In Hotmail, only POP is present and it doesn’t support IMAP.
The biggest advantage of using IMAP is you can access your mail from multiple mail clients and each client detects the change in real-time. Suppose mail server is connected with two different mail clients (let’s say Client 1 and Client 2) on different computers. If the user deletes a message in mail client 1, the change will appear on mail server immediately and also on mail client 2. In IMAP all messages from mail clients and servers are synced with each other.
You can download emails from mail server to your PC using POP. After downloading, the original mail is removed from the server and hence you can’t access it from another computer (Note: In Gmail there is an option to keep the copy of mail in inbox. Thunderbird also provides an option to leave messages on server until you delete them). But there are lots of other options missing (for ex. if you send a message from mail client then you won’t find that message under sent items in your mailbox).