MMX technology is the first implementation of the SIMD (Single Instruction, Multiple Data) concept on PCs, by Intel. MMX is a set of 57 instructions that manipulate several small data at once. For example, using a 32-bit register to load and execute four 8-bit instructions at once. In this example this task would be four times faster than manipulating the four 8-bit data individually.
In order to use this technology the CPU must have this instruction set and the software must be compiled (i.e. written) to use it. Otherwise there is no performance gain.
Other implementations of the SIMD concept that follow the same idea of MMX are SSE and 3DNow.
Introduced with Pentium with MMX Technology processor, this technology is present in all CPUs available today.