GIF stands for Graphics Interchange Format. It's an old but hugely popular image format that can store a short, silently looping animation. Its defining limits are that each frame uses at most 256 colours and it has no sound.
Those limits are why GIFs of real video often look grainy and end up large — photographic colour doesn't fit into 256 shades. For anything longer or more colourful, an MP4 is usually better.
Make a GIF from a video or from images, free and in your browser.