Public transport in Toronto is surprisingly good and much better than what I expected of a North American city. The system is completely integrated and easy to understand and use.
Essentially, it consists of a thick network of buses supplemented with trams (street-cars) combined with a three-line underground (subway) system. There are also some local trains (GO trains) which are mostly for commuters and a typical visitor will not find much occasion to use them.
Each single one way journey on the bus/subway system costs 3 CAD, and in some cases you need to obtain what is called "a transfer" to be able to change vehicles, in others the transfer is seamlessly automatic, without a need for the paper slip. A child ticket costs 75 cents, and all children above 2 years old pay.
In addition to single ticket there are also passes that allow for a freedom of the whole system for a day. These costs 10 CAD per day, and are thus only of use if you are genuinely likely to make more than 3 single journeys a day. However, on weekends and statutory holidays (and that, amazingly, includes school holidays), the pas is good for a whole group consisting of 2 adults and up to 4 children or an adult and 5 children! This is excellent value and during our stay in Toronto we were lucky enough to be able to use it every day as it fell during so called "March break".
Most bus stops and all subway stations have a map of the system as well as a time table, and the map is easy to read and interpret, though it's worth remembering that it's not to scale which means that what looks like a short hop might prove to be a 20 minute journey (and vice versa).
Toronto subway is great: quick, frequent, clean and efficient, the trains don't seem to get very crowded even in the rush hour. The bus often drive into the subway stations or stop directly outside so even in bad weather one stays under cover.
I am not sure ow comprehensive the system is in the far-out suburbs, but wherever we went (and that included central locations, residential areas not too far from the centre as well as attractions further out (the Science Centre and the Zoo) there seemed to be a bus, street-car or subway nearby. The bus stops are very frequent and the stops are announced via the loudspeaker on the bus as well as the subway.
The system seems safe and at night there are designated waiting areas on the subway stations (well lit and near an intercom system for communicating with staff) while women travelling alone can ask the bus driver to stop at any point and not just at designated stops.
Altogether I found the public transport in Toronto efficient, easy to use and very convenient.