Web based e-mail
The term Webmail (or Web-based email) is used to describe two things. One use of the word is to describe a Webmail client: an email client implemented as a web application accessed via a web browser.
The other use of the word is to describe a Web-based email service: an email service offered through a web site (a webmail provider) such as Gmail, Yahoo! Mail, Hotmail and AOL Mail. Practically every webmail provider offers email access using a webmail client, and many of them also offer email access by a desktop email client using standard email protocols, while many internet service providers provide a webmail client as part of the email service included in their internet service package.
As with any web application, webmail's main advantage over the use of a desktop email client is the ability to send and receive email anywhere from a web browser. Its main disadvantage is the need to be connected to the internet while using it (Gmail offers offline use of its webmail client through the installation of Gears.). There exist also other software tools to integrate parts of the webmail functionality into the OS (e.g. creating messages directly from third party applications via MAPI).
Rendering and compatibility
Email users may find the use of both a webmail client and a desktop client using the POP3 protocol a bit incompatible: email messages that are downloaded by the desktop client and are removed from the server will no longer be available on the webmail client. The use of a webmail client in this mode is limited to previewing messages using a web client before they are downloaded by the desktop email client. On the other hand, the use of both a webmail client and a desktop client using the IMAP4 protocol has no such incompatibility: the contents of the mailbox will be consistently displayed in both the webmail and the desktop email client and any action the user performs on messages in one interface would be reflected when email is accessed using the other interface. There are significant differences in rendering capabilities for many popular webmail services such as Yahoo! Mail, Gmail, and Windows Live Hotmail. Due to the various treatment of HTML tags, such as