Since I'm Catholic, people sometimes ask me what's up with dioceses and archdioceses. It's not too complicated; it basically works like sports leagues or the postal system. Here's the map of the 32 Catholic archdioceses, from Wikipedia:
Click on the map for a fuller version.
Each color represents a different archdiocese, and each little segment represents a different diocese. Generally, they get named after the most important city in the area. For example, look at Washington in the upper/left. It is made up of Yakima, Spokane and Seattle. Dioceses get split up, based on geography and population, and each diocese has its own Bishop.
From what I can tell, the difference between a diocese and an archdiocese is basically nil. The Bishop who is assigned to an archdiocese is called an Archbishop, but he doesn't yield power over the other Bishops in his territory except in extreme emergency situations. And even then, he has to get permission from Rome, first. So the title is more honorary than anything else.