x264 is a free software library and application for encoding video streams into the H.264/MPEG-4 AVC format, and is released under the terms of the GNU GPL.

MPEG-4 is a broad Open Standard developed by the Moving Picture Experts Group (MPEG), a working group of the International Organization for Standardization (ISO) which also did the well known MPEG-1 (MP3, VCD) and MPEG-2 (DVD, SVCD) Standards, standardizing all sorts of audio/video compression formats and much more

By its nature the MPEG-4 Standard doesn't aim at standardizing one potential product (e.g. something comparable to DVD) but covers a broad range of Sub-Standards, which Product Providers can choose from to follow, according to what they need for their product

The AVC/H.264 standard defines four different Profiles: Baseline, Main, Extended and High Profile:

Baseline Profile offers I/P-Frames, supports progressive and CAVLC only

Extended Profile offers I/P/B/SP/SI-Frames, supports progressive and CAVLC only

Main Profile offers I/P/B-Frames, supports progressive and interlaced, and offers CAVLC or CABAC

High Profile (aka FRExt) adds to Main Profile: 8x8 intra prediction, custom quants, lossless video coding, more yuv formats (4:4:4...)

Features:

CAVLC/CABAC

Multi-references

Intra: all macroblock types (16x16, 8x8, and 4x4 with all predictions)

Inter P: all partitions (from 16x16 down to 4x4)

Inter B: partitions from 16x16 down to 8x8 (including skip/direct)

Ratecontrol: optional VBV, single or multipass ABR, constant quantizer

Scene cut detection

Adaptive B-frame placement

B-frames as references / arbitrary frame order

8x8 and 4x4 adaptive spatial transform

Lossless mode

Custom quantization matrices

Parallel encoding of multiple slices

Interlacing