Advanced Video Coding (AVC), also referred to as H.264 or MPEG-4 Part 10, Advanced Video Coding (MPEG-4 AVC), is a video compression standard based on block-oriented, motion-compensated integer-DCT coding. It is by far the most commonly used format for the recording, compression, and distribution of video content, used by 91% of video industry developers as of September 2019.
It supports resolutions up to and including 8K UHD.
H.264 is controversial for being patent-encumbered, and hence subject to royalty requirements. In 2013, Cisco is attempting to partially remedy this by producing a freely distributed executable H.264 codec for many platforms for which they have paid the royalty, allowing anybody to download and use it in unmodified form royalty-free. The source code is also openly available, but any altered versions that anybody might create from it would require separate licensing, as would any distribution of the executables other than direct download from Cisco. Thus, the only way to use it as part of a product without additional license fees is to have the product's installer download the executable from Cisco during the install process, rather than including it directly in your own product (whether on disk or downloadable from your site).
HEVC (H.265) has been developed as a more efficient successor to this format, but it is also patent-encumbered. Attempts to create a royalty-free alternative include AV1, Daala, VP9, and Thor.
The H.264 specification does not, in an extremely strict sense, define a codec which fits into a series of bytes; instead, it describes the stream as fitting into a sequence of frames called "network abstraction layer units", which can then be delineated into a raw byte format by the user. The format for putting these into a raw bytestream described by Annex B of the specification apparently is apparently very widely-used.