Wikipedia - is a good starting point, and states:
The phrase "Wine Is Not an Emulator" is a reference to the fact that no processor code execution emulation occurs when running a Windows app under Wine. "Emulation" usually refers to the execution of compiled code intended for one processor (say, x86) by interpreting/recompiling software running on a different processor (say, PowerPC). Such emulation is almost always much slower than execution of the same code by the processor for which the code was compiled. In Wine, the Windows app's compiled x86 code runs at full native speed on the computer's x86 processor, just as it does when running under Windows. And Windows API calls and services also are not emulated, but rather substituted with Linux equivalents that are compiled for x86 and run at full, native speed.