POP Email Accounts
POP stands for Post Office Protocol and is a bit like having a post office box where you go to the post office (connect to the email server), use your key to open your box (username and password) and get your email. Once you have your email it is with you and no longer on the server (at the post office). With a POP email account your email is stored on the server until you collect (or download) it. You will have a certain allocation of storage space for your email in between the times you check and download your email to your computer.
IMAP Email Accounts
IMAP stands for Internet Message Access Protocol and is where your email is stored on the email server and you access it from one or more devices. If setup correctly when you send an email from one device the sent email will also appear in the Sent Items folder of your other devices. This is a great option if you use a mobile device such as an iPhone or Android smart phone.
With an IMAP account you will have allocated storage space like you do with POP email except your messages and any attachments will always be stored on the server. If your email device dies you can simply get another one, enter your details for your email account and your email will be available to you again.
Another thing to consider with an IMAP account is that you will need to make sure you purchase enough storage sapce for your email.