Central Venice is divided by Venice's longest (4km/2 1/2 miles) and widest (30-70m/98-230 ft.) waterway, the Grand Canal. Its 118 islands are separated by approximately 170 canals and connected by some 430 footbridges, mostly stone with iron balustrades added in the 19th century.
Only four bridges cross the Grand Canal: the Ponte degli Scalzi, just outside and to the left of the train station; the elegant white marble Ponte Rialto (by far the most recognizable bridge and, for centuries, the only one), connecting the districts of San Marco and San Polo; the wooden Ponte Accademia, connecting the Campo Santo Stefano area of the San Marco neighborhood with the Accademia museum across the way in Dorsoduro; and, since late 2008, the futuristic Ponte della Costituzione (also known as the Calatrava bridge, after famed Spanish architect Santiago Calatrava, who designed it), which you will see around the corner to your right when you exit the train station.
Venice lies 4km (2 1/2 miles) from terra firma, connected to the mainland burg of Mestre by the Ponte della Libertà, which leads to Piazzale Roma. Snaking through the city like an inverted S is the Canal Grande (Grand Canal), the wide main artery of aquatic Venice. Central Venice refers to the built-up block of islands in the lagoon's center, including St. Mark's, the train station, and everything else in the six main sestieri that make up the bulk of the tourist city. Greater Venice includes all the inhabited islands of the lagoon -- central Venice plus Murano, Burano, Torcello, and the Lido. The Lagoon comprises everything, from the city to the mud flats to the fish farms to the dozens of abandoned or uninhabited islets.
Keep in mind as you wander seemingly hopelessly among the calli (streets) and campi (squares) that the city wasn't built to make sense to those on foot but rather to those plying its canals. No matter how good your map and sense of direction, time after time you'll get lost (GPS directions are notoriously unreliable here). Just view it as an opportunity to stumble across Venice's most intriguing corners and vignettes.
The city is divided into six sestieri ("sixths," or districts or wards). Cannaregio stretches north and east, from the train station to the Jewish Ghetto and on to the vicinity of the Ca' d'Oro north of the Rialto Bridge. To the east beyond Cannaregio (and skirting the area north and east of Piazza San Marco) is Castello, whose ritzy canal-side esplanade, Riva degli Schiavoni, is lined with deluxe accommodations. The central San Marco shares this side of the Grand Canal with Castello and Cannaregio, anchored by the magnificent Piazza San Marco and St. Mark's Basilica to the south and the Rialto Bridge to the north; it's the city's commercial, religious, and political heart. On the other side of the Grand Canal, San Polo is north of the Rialto Bridge, stretching west to just beyond Campo dei Frari and Campo San Rocco. The residential Santa Croce is next, moving north and west, stretching all the way to Piazzale Roma. Finally, the residential Dorsoduro is on the opposite side of the Accademia Bridge from San Marco. It's the largest sestiere and something of an artists' haven, though escalating rents make it hardly bohemian these days.
Venice shares its lagoon with several other islands. Opposite Piazza San Marco and Dorsoduro is La Giudecca, a tranquil working-class place with mostly residential neighborhoods. The Lido di Venezia is the city's sandy beach; it's a popular summer destination and holds a concentration of seasonal hotels. Murano, Burano, and Torcello are popular destinations northeast of the city and easily accessible by public transport vaporetto. Since the 13th century, Murano has exported its glass products worldwide; it's an interesting half-day trip for those with the time, but you can do just as well in "downtown" Venice's myriad glass stores. Colorful fishing village-style Burano was and still is equally famous for its lace, an art now practiced by so few island women that its prices are generally unaffordable. Torcello is the most remote and least populated. The 40-minute boat ride is worthwhile for history and art buffs, who'll be awestruck by the Byzantine mosaics of the cathedral (some of Europe's finest outside Ravenna), whose foundation dates to the 7th century, making this the oldest Venetian monument in existence. San Michele is the cemetery island where such celebrities as Stravinsky and Diaghilev are buried.
Finally, the industrial city of Mestre, on the mainland, is the gateway to Venice and holds no reason for exploration. In a pinch, its host of inexpensive hotels is worth consideration when Venice's hotels are full, but that's about all.
A Note on Addresses -- Within each sestiere is a most original system of numbering the palazzi, using one continuous string of 6,000 or so numbers. The format for addresses in this chapter is the official mailing address: the sestiere name followed by the building number in that district, followed by the name of the street or campo on which you'll find that address -- for example, San Marco 1471 (Salizzada San Moisè) means the mailing address is San Marco 1471, and you'll find it in the San Marco district on Salizzada San Moisè. Be aware that San Marco 1471 may not necessarily be found close to San Marco 1475 and that many buildings aren't numbered at all.