How DivX Works
DivX is a video codec that became famous for a single, era-defining trick: fitting a full-length DVD movie onto a single CD-R. Technically it is an implementation of the MPEG-4 Part 2 Advanced Simple Profile codec, and DivX files were typically wrapped in an AVI container, paired with MP3 or AC-3 audio. Its compression was efficient enough for its time that a two-hour film could be squeezed to around 700 MB at quality most viewers found perfectly acceptable, which is exactly the size of a blank CD.
That capability landed at the perfect moment. In the early 2000s, DVDs held high-quality video but were large, hard drives were small, and broadband was just arriving. DivX let people rip DVDs to compact files they could store, copy, and share, which made it enormously popular, and closely associated with the file-sharing culture of the period. For a few years, "a DivX" was practically a synonym for a downloadable movie.
It is worth untangling the naming, because DivX has a confusing history. There was an early commercial DivX product, then an open-source codec called DivX ;-) derived from a Microsoft MPEG-4 implementation, then a company (DivX, Inc.) that productized it, and a fully open rival called Xvid that grew up alongside it. For practical purposes, DivX and Xvid are very similar MPEG-4 Part 2 codecs that produce compatible files; the differences matter more to history than to playback.
Why DivX Faded
DivX was superseded by H.264, the codec inside modern MP4. H.264 offered significantly better compression, the same or better quality in smaller files, along with broad hardware support, so it simply outclassed DivX's MPEG-4 Part 2 technology. As H.264 and MP4 became the universal standard, the reasons to use DivX disappeared: storage grew, broadband made large files practical, and the newer codec did the job better everywhere.
Today DivX survives mainly in old downloaded video files and the AVI containers that hold them, plus some legacy DVD players and devices that advertised "DivX certified" playback. If you have old DivX/AVI files, converting them to MP4 (H.264) gives you smaller, more compatible files that play on modern phones, browsers, and TVs without the codec-hunting that old AVI files often require.
When You Encounter DivX
You will meet DivX almost entirely in archived video from the 2000s: old movie rips, downloaded clips in AVI containers, or files from DivX-certified DVD players of that era. There is no modern reason to encode new video as DivX, since H.264 and newer codecs are better in every way. The practical action for any DivX file you want to keep using is to convert it to MP4, which modernizes the file and removes the legacy-codec compatibility headaches.
Limitations
DivX's limitations are those of its age and its codec generation. Its compression is dated, less efficient than H.264, so files are larger than a modern equivalent at the same quality. It is usually tied to the aging AVI container, which brings the familiar missing-codec playback problems and weak support for modern features. And support is fading as devices drop "DivX certified" branding. Its historic strength, putting a movie on a CD, is irrelevant now, so DivX is best treated as a format to convert away from.
