Webmail (or Web-based email) is an email service intended to be primarily accessed via a web browser, as opposed to through an email client, such as Microsoft Outlook, Mozilla's Thunderbird, or Apple Inc.'s Mail. Very popular webmail providers include Gmail, Yahoo! Mail, Hotmail and AOL. See also Comparison of webmail providers.
A major advantage of web-based email over application-based email is that a user has the ability to access their inbox from any Internet-connected computer around the world. However, the need for Internet access is also a drawback, in that one cannot access their old messages when they are not connected to the Internet.
There are also software packages that allow an organization such as company to offer email through the web for their associates. Some solutions are open source software like SquirrelMail, BlueMamba, RoundCube and IlohaMail, while others are closed source like the Outlook Web Access module for Microsoft Exchange, Alt-N Technologies' WorldClient module for MDaemon, and Atmail.
Conversely, there are programs that can simulate a web browser to access webmail as if it were stored in a POP3 or IMAP account. They are susceptible, though, to changes in the user interface of the web service since there is no standard interface.
Some providers, such as Mail2web offer web access to other's email servers. This allows web access to mailboxes where the mail server does not offer a web interface, or where an alternative interface is desired.