Xvid runs very fast as it is optimized for the latest CPUs. Recording video from a camera in realtime or playing back HD video smoothly are not a problem. Your computer will still have plenty of power left for your other tasks!
A big advantage of Xvid compressed video files is that they play almost anywhere. The Xvid software runs on many platforms and most popular video players support it. Also, home entertainment devices like TVs or DVD/BD players can handle Xvid video.
The Xvid codec compresses video files to make them smaller. It can compress video at a ratio of 200:1 or more compared to uncompressed video. This makes transmitting the video over a network speedy or saves you a lot of space on your computer's hard disk.
While Xvid compresses video very well, it can do so without causing visible quality loss and by retaining a sharper picture than other codecs do. Xvid is "lossy" compression but aims at removing just those picture details that are not important for human perception. As such, it be seen as a MP3 for video.
Xvid is distributed as free and open-source software. And that means you can obtain it really for free. It doesn't come bundled with adware or crapware. No unwanted bundle software, no malware, no browser toolbars, no bullshit.
The Xvid codec implements the ISO MPEG-4 standard. Conforming to a standard makes videos compressed with Xvid future-proof and further ensures that the resulting files can be played back or edited also with third-party software.
The availability of the Xvid source code allows "porting" the Xvid software to new platforms and processors easily. Over time, Xvid also already got ported to many different CPU types and operating systems including Windows, Linux and Mac.