The link has a description of what it is, and what features it will have (so far). Based on the screenshots in the link, it's basically an browser version of Outlook:
Windows Live Mail will be the successor to Hotmail, which was founded by Jack Smith and Sabeer Bhatia, and commercially launched in 1996 as one of the first webmail services on the Internet. It was bought by Microsoft in 1997 and, by 1999, had reported more than 30 million active members. In 2004, Google announced their own brand of webmail—Gmail—boasting over 1GB of free storage and a quick, user-friendly interface. This was the beginning of a major revamp of all webmail services, including significant increases of storage and new interface designs, currently being slowly released to beta testers. Windows Live Mail Beta is Microsoft's answer to the webmail revolution, and its main competitors include Yahoo! Mail Beta, and Google's Gmail Beta.