With developer previews, it's always a good idea to keep from flashing it onto your daily driver. They are often times early, and unfinished versions of software, whose purpose is to help developers get their apps ready for an upcoming release and to provide feedback to Google (primarily about bugs). This year, Google released the Android N Developer Preview a bit earlier than usual. Throwing it at us in March (instead of at Google I/O), and then throwing the second one out in April. We are due to get a few more before the final release in the third quarter. The first developer preview was classified as an "Alpha" release, and the second was classified as a "Beta" release.
However, it now appears that they have changed the classification of the second developer preview, bringing it back to alpha status. No one really has a clue as to why this was changed, but according to the definition for alpha and beta, it appears that because many of the features are not in Android N, or are not working, Google had to reclassify the update as an alpha release. For many of us, that doesn't mean much, especially since we should be getting the third developer preview tomorrow at Google I/O.
Now that the second preview has been reclassified, let's go over the schedule for previews on Android N. With preview 1 and 2 being alpha versions, preview 3 is the first beta. Preview 4 is when the final APIs will be available, as well as the official SDK and Google Play Publishing begins for Android N compatible applications. Preview 5 is similar to a release candidate, which Google quotes as being "near-final system images" and then finally we have the final release. Which should put us around August or September for the final Android N release. Google still says that it is "Q3" however. Which is likely to keep from giving out a hard deadline, as these things do change quite often.
Android N isn't a huge overhaul like Android L was, two years ago. But the new version of Android does have quite a few changes included. Although the big one is multi-window, something we saw in Android M last year, but was pulled before the final version.