Mars was the internal codename for the project that became known as MSN Explorer (the shipping name). Mars was mainly an interface design project, and became the core design for the new UI of the upcoming consumer OS,Windows XP. As you can see, they are very similar in concept; the logon screens are almost identical. This design is focused towards consumers and ease of use.

The Mars project was formerly known as Haley, and was very big inside of Microsoft. It had these really neat Alien posters that said "You've got Mars!" in spooky cheesy sci-fi flick letters. It was really quite cool.