Whichever way you reach Venice, whether it be by train, car or airplane, when you're there you'll have to use one of the following means of transportation:
Your legs
The cheapest of the lot, very convenient because they take you exactly where you want to go, do not cost anything and keep you fit. Since Venice isn't that big, you can get anywhere in at most one hour and a half. If you go to Venice often, you tend to assume what is commonly known as the 'Venetian walk', which is not unlike that of a long-distance runner. If you manage the Venetian walk, you can get from the Station to San Marco in little more than half an hour. Remember, the most vital thing to have in Venice is a map, since even if it is not difficult to guess the right direction if you know the city a little, if you don't you can get lost and waste a lot of time.
Vaporetto or Battello
These are the two names in Italian for the small public boats (capacity 120 approximately) that are Venice's buses. Vaporetti have several roues, all of which either circumnavigate the city or go through the Canal Grande, the main channel in Venice, which starts at the Station, passes underneath the Rialto bridge and ends in the lagoon in front of San Marco. Vaporetti are not that cheap (not even for residents), but are great if you're tired, you have to go far or have heavy baggage with you. In peak seasons, it can be difficult to get into one because they're often full.
Water taxis
Mostly used by Hollywood stars when coming to the Venice Film Festival, they are extremely expensive. Advised only to very big groups willing to share the expense. And Tom Cruise.
Gondolas
Gondolas are not a real means of transportation: they're the Venetian equivalent of the nice little carriages pulled by a poor-looking horse that take you to make insignificant sighseeing trips around all tourist cities in the world. If you go for a trip with a Gondola, expect to have the Gondoliere (the guy who's rowing) sing out-of-fashion songs, the most popular of which is apparently "O' sole mio", which doesn't have anything to do with Venice anyway, since it's in Naples dialect. If you wish to look ridiculous and be the victim of an extortion, go ahead and do it. It's very romantic.