The bus terminal (now known as Gare d'autocars de Montréal, previously called Station Centrale and Terminus Voyageur) is located on Berri street, corner of de Maisonneuve (505, boul. de Maisonneuve East) and is serviced by many companies, each with different routes. A limited selection of tickets can be bought online here: stationcentrale.com . For more complete information visit the following web sites:
Orléans Express for Trois-Rivières, Quebec city, Rivière-du-Loup (connections to the Maritimes) and Gaspé, etc
Sherbus for Sherbrooke and Magog, etc
Greyhound and Adirondack/Pine Hill Trailways for the United States. You can lookup fares and schedules on the Greyhound web site, you just have to specify where you are coming from and that you are going to Montreal PQ, but not all US cities have a direct route to Montreal and they don't give you the indirect connections. But most large northeastern US Cities, like New York, Boston or Washington DC, have a direct route to Montreal.
The bus terminal is directly connected to the Berri-UQAM metro station. You don't even have to go outside, there are some stairs that go down to the metro station. From there you can take the orange, green or yellow lines (see the metro & buses section). For informations on schedules and prices, you can call 514-842-2281.