A processor core or just core is an individual processor within a CPU. Today many computers have multi-core processors that means a CPU contains more than one core. In early years, computers only had a single core, and in the 2000s, as processor clock speeds began plateauing, CPU builders needed to find other ways in order to increase processing performance.
Primarily, they attained this by putting many processors on high-end computers. While this was effective, it added cost to the computers and the multi-processing performance was limited by the bus speed between the CPUs.
By combining multiple processors on a chip, CPU manufacturers were able to increase performance more professionally at a lower cost. The individual processing units are known as cores rather than processors.
In the mid-2000s, dual-core and quad-core processors began that replaced multi-processor. While only high-end computers contained multiple-cores and today nearly all computers have multi-core processors.