At the end of the long bridge over the lagoon that connects Venice to the mainland, and just across the Grand Canal from the train station (linked since 2008 by the Ponte della Costituzione), Piazzale Roma is Venice's main transport hub, with parking garages and a bus station with connections to mainland towns and to Marco Polo Airport and Treviso Airport. It has a very popular left luggage (Deposito Bagagli) office; long queues are not uncommon.
A word of warning: Eating or drinking at any place in the vicinity of Piazzale Roma (except the excellent train station bar) probably won't kill you, but it won't be very good and it will certainly be overpriced.
vaporetto: Piazzale Roma
Images by: Matthew Black, Creative Commons License