Welcome! Come back again to visit sometime; I think you will notice some physical changes.
The GTA (Greater Toronto Area) generally refers to Toronto and the surrounding regions (formerly called counties, many years ago) of Halton, Peel, York, and Durham. It's an informal designation. The City of Toronto has a population of just over 2.5 million, per 2006 census numbers. The GTA is roughly 5 to 5.5 million, depending on exactly where you draw the lines.
Previous to 1998 the City of Toronto and other municipalities in Metropolitan Toronto each had their own mayor and council. These were "lower tier" municipalities. The Municipality of Metropolitan Toronto (the "upper tier" level) had its own council, made up of members of the lower tier councils. All of these are now abolished, and one "single tier" council is in place.
Relationships with the Provincial and Federal governments can be problematic, as you might gather from some posts on this forum, newspaper articles, etc. The basic problem is funding, especially for infrastructure such as transit. The City of Toronto has recently been given some new taxing powers which it did not have before, and debate is under way now as to whether some new taxes should be imposed. A scan of some newspaper columns would reveal more.
Could go on at great length; I hope this quick comment will be helpful.