Built in PC Emulator in Mac OS X?
Actually there isn't... What Mac OS X comes with is a utility called BootCamp, that allows you to create a separate partition on the hard drive, into which Windows can be installed the same way you would install it on a PC. After that you can boot into Mac OS X or Windows, but not both at the same time.
Emulation is not included built in, but is freely available. I use Oracle VirtualBox, which allows me to run Windows in a window on top of Mac OS X, and best of all it is free! Paid for "hypervisors" like VMware Fusion and Parallels Desktop offer more features, improved performance and better integration into the Mac OS, but for most people the extra cost is not worthwhile, in my opinion.
There is, however, and emulator built into Mac OS X called Rosetta, which is used by Mac OS X to run legacy operating system components and any applications which are compiled only to run on PowerPC based Macs. This is used invisibly by the operating system, and certainly not what you are enquiring about.