Despite it's place as a city of tourists (and the stereotypical tourist penchant for local drink and dance), Venice is hardly a pub crawl or rave destination. Which is not to say that Venice's contributions to world culture are limited to its canal-ways and churches; if you're one for cocktails, then the ubiquitous spritz is an invariable go-to that originated in this venerable city. Also, despite the fact that you'll not have a bar street or any multi-tiered superclubs to sate your nightlife interest, there are plenty of establishments spread out around the city that will give you a chance to kick back and socialize in one of the most unique parts of the world.
Most Venetian drinking is of the casual variety, done during their evening bacari (i.e. happy hour) as the cicchetti (a dinner of small snack plates) is served. The one drink that has become synonymous with Venice is the spritz, a carbonated beverage that mixes liqueur (Select is a popular choice), white wine (Prosecco), and sparkling water. Once considered an "old folks" drink, the spritz has seen a revival in recent generations and is a must when kicking back on cafe patios or along the the many piazzas of the city. The Veneto region is a high producer of white wines in general, particularly the aforementioned Prosecco and Pino grigio varieties. Be aware of cheap food prices displayed outside of restaurants; you may find yourself paying exorbitantly for your drinks. Just inquire before ordering.
As far as concentrations of nightlife go, your best bets are in the districts of Dorsoduro or San Polo, not, as some would expect in San Marco, where Venice's most famous piazza is located. Campo Santa Margherita in Dorsoduro is the place to hang out for local university students; plenty of bars, osterias, and cafes stay quite busy on the weekends on this large, open square, making it an enticing stop from afternoon until night. The district of San Polo is also a quality option, particularly for its proximity to the highly sought out Rialto Bridge and the nearby market. Al Merca (outside service only) and Ancora are happening little wine bars in the area, while Muro Venezia is more the beer and football vibe. If you're really hankering for a strip of nightlife joints, then Lido di Jesolo, which is a short ferry ride or a doubling back along the mainland by car, is your best bet. With cocktail lounges and bars that look out on the Adriatic, the party vibe is evident, particularly at Piazza Giuseppe Mazzini.
For further information on Venice's nightlife, take a look through our dedicated pages.