A multi-core CPU is a computer processor which has two or more sections. Each section of the chip executes instructions as if it was a separate computer. The actual processors are still on one chip. On this chip every core looks mostly like the other. They are several mostly independent cores which work together in parallel. A dual-core processor is a multi-core processor with two independent microprocessors. A quad-core processor is a multi-core processor with four independent microprocessors. As you might be able to tell from the prefix, the name of the processor is based on the number of the microprocessors on the chip.
In the 20th century most computers used single-core processors and not multi-core processors. Multi-core solutions were only used in unusual cases. The usual way to make a computer faster was to increase the clock rate. But at a frequency about 4 GHz the CPU would get too hot and take a lot of electricity. This was the point when multi-core processors became more important. Therefore, the demand for multi-core processors increased. In the second half of 2006 the best processors were dual-core processors. Since 2006 the development has gone on, so that the new processors get four, eight, or more independent microprocessors. Today, single-core processors are not used in new personal computers, but they remain popular in embedded systems which do not need a fast processor.
The following operating systems support multi-core processors