The Scobleizer LonghornBlog "What is Longhorn? Really short version before I hop on a plane to go home:
a) a new User Interface. (code-named Aero). Nicer looking, more productive.
b) a new file storage system. (code-named WinFS) Easier to search for files. Allows for new views on files, people, and data. Lets you build “associations” between data and people.
c) A new set of APIs. (code-named Avalon, now called WinFX). Much more productive for programmers. Offers new user interface capabilities.
d) A new way for software/computers to talk to each other (code-named Indigo). The next version of SOAP and Web services.
e) A new way to communicate (code named RTC). The next version of collaboration and IM functionality."