The CPU in Macs have had a long history, changing between different companies. Today the CPU is now done by Intel.
The Apple I and Apple II used CPUs from a company called MOS. This was because they were very cheap for the company and they worked brilliantly. The company changed to Synertek due to company takeovers
The Apple Lisa used CPUs from Motorola (yes the ones who make phones), which made the clock speed a little faster. The Motorola CPUs were used right through the Macintosh range until the Power Macintosh.
In early 2006 Apple announced that the new iMacs at the time were going to have Intel dual-core based processors. This has stayed since.