The land bus system of Venice—or, more accurately, how Bus #5 usefully connects the airport, parking lots, and Mestre with Venice
That's because buses—quite obviously—are only useful in Mestre, the landlubbing side of Venice, an area which smart travelers avoid entirely except to pass through en route to the historic, lagoon-locked Venice of canals.
Unless something goes terribly wrong with your vacation plans, there is only one bus you should even have a remote chance of using:
The Venice airport bus #5
Bus no. 5 goes from Venice's Marco Polo airport and stops a few times (including, handily, at the S. Giuliano parking lots in Mestre) before crossing the bridge to Piazzale Roma—the only part of downtown, insular Venice you can get to by land vehicle.
However, unless you happen to be staying very close to Piazzale Roma (unlikely), you'll then have to take a vaporetto to get to your hotel, adding another €7.50 ($8) to the bus ticket and thereby making it far easier to pay a tiny bit less (€15 ($17)) for an Alilaguna ferry direct from the airport into the historic center Venice. » more
Also, this bus used to cost €1.30 ($1), so it was a modest savings. Now it costs €8 ($9)—which is exactly what the express shuttle bus from the airport directly downtown costs, so there is no earthly reason to take this slower, local bus.
Excpet for that airport bus #5, all other Venice buses cost €1.50 ($2) for a ticket valid for 75 minutes.
You must buy bus tickets ahead of time from a newsstand (not all sell them, but those near bus stops—and in airports and train stations—usually do) or a tobacconist (shops identifiable by a white-on-brown "T" sign).