If you are flying direct to Canada, the main international airports in the east are in Halifax, Toronto and Montreal and in Winnipeg, Edmonton, Calgary and Vancouver in the west.
Many visitors, however, enter Canada from the US by plane, train and bus. Air Canada has the most flights between the two countries but most major US and Canadian carriers also fly daily between the major cities.
The three major rail routes from the States are New York to Montreal, New York to Toronto and Chicago to Toronto. The Greyhound network also connects with most of the major cities in Canada but this trip will involve a bus transfer at the US-Canada border.
If you are flying to Canada from outside the US, British Airways, Air France, SAS, Lufthansa, and KLM all have non stop flights. Air Canada and Canadian Air also have international flights to and from most cities in northern Europe and Asia. Flights between Australia, New Zealand and Canada usually connect through Los Angeles or Honolulu.
Once you make it to Canada, it is inevitable that if you want to see more than one area, you are going to have to do some amount of internal travelling. Doing this on land is cheaper and more interesting. Unfortunately, it does also eat up quite a bit of your holiday time.
The majority of rail traffic in Canada is carried by VIA Rail. These trains provide sleeping cars, bedrooms and roomettes for cross country travel. This service travels to almost all of the major cities but the service is a lot less frequent than it used to be. If you do decide to travel a lot by VIA Rail, you should purchase the Canrail pass which will cost you $569C for 12 days of travel over a thirty day period during peak season and $369C in off-peak (after Oct 15).
The bus service is the most extensive public transport system in the country and is also less expensive than the limited train service. Greyhound Canada is the largest coach operator and the Canada Coach Pass allows unlimited travel over a period of consecutive days (7, 15, 30 or 60 days). Prices range from $249C to $599C and the service is a great deal more frequent than the aforementioned train.