There are three bus companies in Plymouth - City Bus , First Bus and Target Travel - they run completely different buses on different routes. The tickets for these buses are not interchangeable. The custom is to wait at the bus stop and then when your bus approaches, signal by putting your arm out for the driver to stop.
Both bus companies sell bus passes which are useful if you use the bus every day. First Bus weekly passes for more than 1 week can be bought on First Buses and also at Bretonside Bus Station. For City Bus passes you should go to the City Bus office in Debenhams store (on the third floor) in the city centre.
For up-to-date public transport time tables contact Traveline.