From Instagram blog:
Today, we’re thrilled to introduce Video on Instagram and bring you another way to share your stories. When you go to take a photo on Instagram, you’ll now see a movie camera icon. Tap it to enter video mode, where you can take up to fifteen seconds of video through the Instagram camera.That's right, the set of filters you can use for videos are not the same as those for photos.
You’ll also find that we’ve added thirteen filters built specifically for video so you can keep sharing beautiful content on Instagram. When you post a video, you’ll also be able to select your favorite scene from what you’ve recorded as your cover image so your videos are beautiful even when they’re not playing.
As stated in the blog entry, you retain your intellectual property rights over your videos, just like your photos.
Instagram has also included a feature named Cinema to stabilise video recording, so that your video appears beautiful and professional.
Instagram with 15 second video requires version 4.0 of the app, so please head to Apple App Store or Google Play to download it now.