Greater Toronto Area
The Greater Toronto Area (GTA) is centered on the city of Toronto, in Ontario's Golden Horseshoe. It is not legally defined but is used as a catch-all reference, and extends west to include Mississauga and Oakville, and east to include Ajax, Pickering, and Whitby.
By public transportation
GO Transit runs trains and buses between Toronto and its suburbs. Most train routes only operate during rush hour and are replaced by coach services at other times. The exceptions are the Lakeshore East and West rail lines, which run all day and on weekends from Aldershot to Oshawa. GO Trains as well as a few GO bus routes run to Union Station in downtown Toronto, which is connected to a subway station with the same name. Most GO buses run to Yokdale or York Mills stations on the Yonge-University-Spadina subway line or Scarborough Centre on the Scarborough RT line.
By public transit
GO Transit  operates the commuter transit services between the different regions of the GTA. It operates 7 commuter rail lines, all coming together at Union Station at the base of Toronto's financial district. All the rail lines other than the Lakeshore West and Lakeshore East operate into Union Station during morning rush hour and out of Union Station during evening rush hour and run only on weekdays. The two lakeshore lines run all day from around 6 am to around 1 am on weekdays and weekends. Buses replace the rail lines while they're not running, most GO buses operate out of Yorkdale and York Mills subway stations at the north end of Toronto and several use the Union Station bus terminal, across the street from the train station.
Within each region of the GTA, local municipalities operate transit services. The TTC  is by far the largest and serves the city of Toronto, it runs all the buses, streetcars and subways within the city. York Region Transit operates buses in York Region, to the north of Toronto. YRT also runs 5 bus rapid transit lines, known as VIVA; two of these lines operate only during rush hour and four of them connect to subway stations. Mississauga Transit serves Mississauga to the west of Toronto. Most of its services are centred on the Square One shopping centre and many of its routes connect to the Bloor-Danforth subway line at Islington station. There are several express routes that run on expressways, many routes connect to GO train stations and route 7 connects Square One with Pearson International Airport.
Some options for further travel include: