According to this image (via Reddit), there's four story chapters and four multiplayer maps coming in the beta for Destiny, which is due to start in Europe on July 17 on PlayStation platforms, and a week later on Xbox.
The image confirms four chapters, four multiplayer maps, a new co-operative Strike, and "a huge world to explore with your own unique Guardian". Those who grab a beta code will actually receive three codes (you redeem your code here), so you can take a couple of buddies with you on the three-player Strike missions.