How do Emulators really work?

[Flushie]

Alright, so I am now asking a question because I want to start doing some programming in them.

I am wondering how Emulators work for World of Warcraft, basically I understand that WoW reads the packet and understands how to handle it, but with the server how does it handle some things like say you got the weather system, how does it work, this may be a retarded question, so if you feel like you need to flame, dip you head in water.