Original: Webmail (or Web-based e-mail) is an e-mail service intended to be primarily accessed via a web browser, as opposed to through a desktop e-mail client (such as Microsoft Outlook, Mozilla's Thunderbird, or Apple Inc.'s Mail).
Simplified: Webmail (or Web-based e-mail) is an e-mail service which is usually accessed through a web browser, instead of an email client such as Microsoft Outlook.