Videos/sounds/animations in a web page, and the popular forms of them.

They're mainly used for aesthetic values, and to help improve the visitations to that web page. Most people enjoy having something that is pleasing to their eyes, as well as that entertains.

Popular forms of...

Video - .flv, Flash Video File-Flash-compatible video file exported by the Flash Video Exporter plug-in (included with Adobe Flash) or other program with FLV file support; consists of a short header, interleaved audio, video, and metadata packets. Mainly used because flash is a simple form of video to use, and is the most widely used form for videos.

Sound - .mp3, mp3 audio file- commonly used to store music files and audiobooks on a hard drive; may provide near-CD quality sound with 1/10th the size of a WAVE file. Mainly used as most people enjoy having their music on their mp3 players/iPods, as to on a CD or a computer. Allows for easier mobility.

Images - .gif, Graphical Interchange Format image File- Image file that may contain up to 256 indexed colors; color palette may be a predefined set of colors or may be adapted to the colors in the image; GIF images can also be animated and saved as "animated GIFs," which are often used to display basic animations on websites.

    .jpg, JPEG image file- commonly used for storing digital photos since the format supports up to 24-bit color. JPEG is also a common format for publishing Web graphics since the JPEG compression algorithm significantly reduces the file size of images. Compression noticeably reduces the image quality.

These two are the most popular to work with, as they are easy to edit, or put into a website/web page.