There are several bus routes running around Sopot. Some of them lead to the neighbouring districts of Gdańsk and Gdynia.
Bus transport is managed by two companies: Zakład Transportu Miejskiego in Gdańsk and Zakład Komunikacji Miejskiej in Gdynia. They issue their own tickets each, but within limits of Sopot you can use any carrier's ticket in any bus. Crossing the city border does not require validating an additional ticket, however remember to travel to Gdańsk with a Gdańsk ticket, and to Gdynia with a Gdynia ticket. A one-off ticket costs 3,80zł, it can be bought at kiosks and at the driver's on the bus. On the bus, it should be validated immediately when boarding.
Although there are many lines, most of them do not run more often than once per hour. So it is rarely worth the wait to drive 2 or 3 stops, especially within the city center. Routes and timetables are not very comprehensible. The most difficult to grasp is the timetable of the route 144, which every run changes to one of its three variants of the route.