The human eye is more sensitive to brightness changing than to color changing. Therefore in YCbCr color space, the Y component contains more details than chroma components (Cb, Cr). In practice, reducing resolution of the chroma components causes a reduction of data amount without loosing visual perception quality.
In JPEG compression chroma components data are reduced by special sampling methods.
In 4:2:0 sampling method the chrominance components are reduced by taking the mean value of each 2x2 block. It means that for four samples of the luminance there is one sample of chrominance (Cb and Cr separately). The figure below shows 4:2:0 sampling diagram.
In this sampling method the chroma components are reduced only in horizontal dimension. It means that there is one chrominance sample (Cb, Cr separately)for two luminance samples. Sampling method is shown below.
This method of sampling is straightforward and does not reduce chrominance data. For each luminance sample there is one chrominance sample (Cb and Cr separately).
In this method the chrominance data are completely removed. There is only luminance component.
The minimum coded unit (MCU) is a collection of 8x8 pixels blocks, which is required for representing data in specified sampling method. For example, if sampling method is 4:2:0, then MCU is a collection of four Y blocks, one Cb block and one Cr block. If sampling method is 4:2:2 then MCU is a collection of two Y blocks, one Cb block and one Cr block. The 4:4:4 is a collection of one Y block, one Cb block and one Cr block. Below figure shows diagram for 4:2:0 sampling.