SSE stands for Streaming SIMD Extensions.

Introduced by Intel in its line of Pentium III processors, SIMD technology allows for "Single Instruction Multiple Data." While older processors could only process one data element per instruction, SIMD technology allows instructions to handle multiple data elements, making processing much quicker.

SSE's use of SIMD technology allows for data processing in applications such as 3D graphics to benefit greatly from the use of SSE.

In contrast to MMX, a fairly similar technology, SSE registers have much larger bits, allowing SSE to work even quicker than the advanced MMX technology.

SSE has continuously been improved upon and now SSE2 and even SSE3 technology have been developed. These technologies even further improve upon the concept of multiple data elements being processed in single instructions.