Flynn's taxonomy is a way to classify computer architectures based on the number of instruction streams and data streams available.
SISD: Single instruction stream single data stream
The classic PC with one processor that do one thing at a time.
SIMD: Single Instruction Multiple Data
One thread with wide execution data. Example x86 SSE/AVX.
MISD: Multiple instruction streams, single data stream
MIMD: Multiple Instruction Multiple Data
Multiple independent threads. Example multicore CPUs.
SIMT: Single Instruction Multiple Thread
Multiple lockstep threads. Ex GPUs. An instruction is issued for an entire group of threads at the same time. Thread carriers out the operation on it's own arguments.
SPMD: Single program, multiple data streams
MPMD: Multiple programs, multiple data streams