Twitter GIF are VIFs. They're regular MP4 files that basically play in a loop.
Why does Twitter convert your GIFs to MP4? Turns out it's all about size and control.
Sean Creeley of Embedly has a great explanation on his blog, in which he compares the file sizes of a GIF and an MP4 file. A GIF, says Creely, can be five times the size of an MP4 file. That's because an MP4 can take advantage of all sorts of compression techniques, unlike a GIF.
Also, GIFs are unpausable and videos are not. You can play, pause and seek at will. And you can even slow them down. Because let's face it, nothing beats a slo-mo GIF — even if technically, on Twitter, it's not a GIF. [Embedly]
[GIF via BuzzFeed]