IMAP (Internet Message Access Protocol) is perfect if you are using the same email account on multiple devices such as a desktop computer, laptop, mobile phone, and Web-mail. The status of your email will be synced accross your devices if set up correctly, e.g. if you read it on your tablet then it will be marked as read on all your devices.
POP3 (Post Office Protocol) downloads the emails to your web client and then (in most cases) delete it from the server after a set period. Your emails might get lost if the same address is used on different devices as the device you read the email on first might delete the email on the server before the other devices download it. This is only recommended when you are using only one device.
