Web browsers allow users to ask for, receive and display pages that have been formatted (typically) in a markup language such as HTML. HTTP is the protocol that enables the request and transmission of web pages between browser to server and is currently at version 1.1.
There are a number of browsers in use (from a variety of different sources). IE is currently at version 7 after many years of very little development. Both Firefox and Safari are open source projects, which means that both the source and executable code are widely available. Firefox and Safari have best support for web standards, supporting more of CSS 2 than IE.
There are other browsers as well (Opera, Konquerer).