Web bug (also Web beacon):

A 1 pixel-by-1 pixel image tag added to an HTMLmessage and used to track open rates by email address. Opening the message, either in the preview pane or by clicking on it, activates the bug and sends a signal to the Web site, where special software tracks and records the signal as an open. Webmail (also Web mail): Any of several Web-based email clients where clients have to go to a Web site to access or download email instead of using a desktop application. Some examples are Gmail, Yahoo! Mail and Hotmail.