POP3 only supports 1 PC. With IMAP you can download email to several PCs or devices, it keeps the emails on the server until you delete them. Here's a longer explanation from Domain Monster: If you decide to configure your email client or mobile device to connect to your mailbox using POP3 then what happens is that the client or device contacts the mailbox and allows them to download the contents of the Inbox to their own local storage. The mail then will only exist on that local storage and would not be available to download via another client or device. It is however worth noting at this point that a number of clients and devices do allow you to enable a setting to ‘keep a copy of the email on the server’ this would then allow them to be download via a second client if required.
When using IMAP with a client or device they will simply connect to the mailbox and display the full mailbox to you without actually downloading the content to local storage. This has the benefit of allowing you to manage your mailbox from multiple clients and devices and seeing the same content.
If you think about your mailbox as a post-box full of letters, and your mail clients and devices as postmen it can become a little easier to understand the basic fundamental differences between the two protocols.
When using POP3 a postman would visit the post-box and empty the contents and take it away with him. If a second postman then arrived he would only have access to the letters that had been placed into the post-box since the first visit.
When using IMAP a postman would visit the post-box and take a copy of the letters and take those copies away with him, then when a second postman arrived he would do the same and still be able to see all of the letters placed into the post-box.