The first "Android L" developer preview arrived shortly after Google I/O, and was updated in early August with Google Fit support (and little else). It's unclear how much new stuff is in these latest preview images, but with just a couple of weeks left until the new Nexus devices start shipping, we're hoping for a more complete look at the next version of Android.
As always, these are preview builds intended to help developers tune their apps for the next version of Android. It's absolutely not the final version of Lollipop, and chances are there'll still be some bugs lurking. So we'd advise against flashing these preview images to your main device just yet.
Keep it locked to AC for full coverage of what's new in the latest Lollipop preview later today!
Source: Android Developers