From Second Life Wiki
A "First Look" differs from our Preview Grid viewer in the following important way: The Preview Grid viewer connects to a special test grid that is separate from the main grid. On the Preview grid, for instance, you can spend Linden Dollars inside the testing grid without actually affecting your true L$ balance. However, the First Look viewer connects to the main grid. If you're using the First Look viewer, you're getting a chance to try out some interesting new features, but remember that every transaction or interaction (like spending Linden Dollars) is taking place in the "real" Second Life, just as if you used the normal official viewer.
Why should I install the First Look viewer?
Good question! It's to get a sneak-peek at a nifty new feature (or features) while we test and tweak it for the maximum quality possible.
One important thing about the First Look viewer is that these new features are still undergoing some revisions. If you're looking for information on those new features, you might also find something by searching this Second Life Wiki that describes the development project in more detail.
After a feature is shown in First Look and then undergoes more development, it will often reappear in a Release Candidate viewer, which is the last step for public testing before that feature can become an official part of the Second Life software.
Where can I download a First Look?
See if any First Look viewers are listed on our Downloads page.