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.
It is also linked to Tronchetto (with the cheaper car park) and the cruise ship and ferry terminal by the Venice People Mover overhead transit, although this is less used now as many large cruise ships dock elsewhere.
vaporetto: Piazzale Roma
Image by Matthew Black, Creative Commons License