In a nutshell
Flynn's taxonomy sorts computers by two questions: how many instruction streams (separate command sequences) and how many data streams (separate pieces of data) it handles at the same time. That gives four boxes: SISD, one of each — a plain single-core CPU; SIMD, one instruction applied to many data at once — GPUs and vector units; MISD, many instructions on one data stream — rare; and MIMD, many of each — multicore CPUs and clusters.