4:4:4 Y'CbCr
Each of the three Y'CbCr components have the same sample rate. This scheme is sometimes used in high-end film scanners and cinematic postproduction. Two links (connections) are normally required to carry this bandwidth: Link A would carry a 4:2:2 signal, Link B a 0:2:2, when combined would make 4:4:4.
4:4:4 R'G'B' (no subsampling)
Note that "4:4:4" may instead be referring to R'G'B' color space, which implicitly does not have any chroma subsampling at all. Formats such as HDCAM SR can record 4:4:4 R'G'B' over dual-link HD-SDI.
4:2:2
The two chroma components are sampled at half the sample rate of luma: the horizontal chroma resolution is halved. This reduces the bandwidth of a video signal by one-third with little to no visual difference.
Many high-end digital video formats and interfaces use this scheme.