When using an email client on your computer like Mozilla Thunderbird or Outlook you should configure it to use IMAP mail instead of POP if possible. The biggest difference between these two is that POP mail is designed to allow the mail to exist on one computer at a time. IMAP is designed to synchronize your mail across multiple computers.
Here are the two main reasons you should strongly consider choosing IMAP over POP when possible.
Mail messages are synchronized across computers: When you check your mail from multiple computers using IMAP you are able to create folders and move messages on one machine and have those folders duplicated and all messages moved on all your other computers. This is the single biggest advantage of IMAP.
Sent messages are transfered to all computers: When using more than one computer to manage your email using POP, it was common to write a message from one computer and not be able to find it on your other computers. Because mail is synchronized across all machines when using IMAP, you are always able to see all of your sent email from any machine.
Not all ISPs or domain hosts allow you to use IMAP. This limitation is set because as long as you have mail in your inbox or any folders on your computer, those messages are also taking up space on the mail server. That is space that the ISP or webhost could be charging someone else to use. They would rather you use POP because once you download the message to your computer, it frees up space on the mail server.
If you always use a webmail interface to check your email, then this is not much of an issue. Webmail is a way to check your mail from services like Gmail and Yahoo! mail. Your ISP may also have a wembail interface to allow you to check your email. Webmail can be POP or IMAP, it depends on the service. When using webmail you don’t have to worry whether the mail is synchronized across multiple computers because you never actually download your mail onto a computer; you use your web browser to look at the mail that is on the mail server.
Using an email client on your computer is usually a better option than using webmail. Having a copy of your email on your computer makes it possible to read and respond to email even when you don’t have an Internet connection. Your messages are sent the next time you connect to the web. You cannot use email offline if you use webmail. However, it is possible to set up your mail client to check your email from services like Gmail which will then give you all the advantages of using a mail client.