We won't spend a lot of time going over what Movie Creator is, because it is essentially the same as the rest of the pack: You can pick out a bunch of your favorite clips and images, choose a song you like and presto -- the app automagically produces a short video that you can share on Twitter, Facebook and other services (or keep to yourself, you selfish person). Each company does it a little differently, so it's no surprise that Microsoft is adding its own flavor to the app genre.
Movie Creator doesn't impose any limitations on the length of the clip, and you can choose up to 25 videos and stills (and Cinemagraphs, on compatible devices) to include in the reel. Once you've picked out the ones you want, you're presented with a linear timeline, each clip or image represented by a square. You can rearrange which order the pics are shown in, the music playing in the background (you can use either Microsoft's music selections or your own library), the duration of each, add captions either on or in between images, rotate them, use filters to change how they look and you can even dictate if and how each clip pans across the screen (it lets you determine where each one starts and ends its animation).
There are also plenty of preset themes, but there's quite a few more available for download if you don't see one that fits your needs. Themes have been done many times before, but Microsoft made a deal with Dreamworks that adds a unique element to them: You'll be able to grab themes featuring characters from movies like Madagascar and Kung Fu Panda. In my preview build, these options weren't available, so hopefully they'll show up in the final software.
Finally, you'll be able to change the quality of the finished product. Depending on your desired size and network speeds (in case you want to upload and share it), you can choose between low, medium and high quality, and you can even select how many frames per second you'd like the reel to finish up in -- you can go up to a maximum of 30fps, so keep that in mind. It appears that the video resolution tops out at 1080p quality as well. Once you're done, the file gets saved on the phone regardless of your sharing preferences, so it's going to take up space on your phone or tablet by default. Since Movie Creator is starting out in beta, it'll likely see some new features sprinkled in between now and its final release.
Update: Microsoft reps have confirmed that the app is only available for Lumia devices, Surface tablets and the Lumia 2520, not all Windows Phones and tablets.