Boston has a thriving nightlife and is known to be a 'drinking' town. There are many venues that cater to college students, businesspeople, sports fanatics, and many others. Bar Hopping is very easy and commonly done
That said, if you're taking the subway or buses back to your hotel, you may have to call it a night early lest you miss the last train by mistake. And if you have people under 21 with you, you're going to have trouble finding a place that will let your group in; pretty much every bar/club in and around town is 21+.
With a large Irish population, Boston has a number of very good Irish pubs. Many tourists look for an authentic "Boston Irish Pub". A good rule of thumb is if the establishment has a neon shamrock in the window, it is not an authentic Irish pub. For nightlife and club listings look for "Stuff @ Night" or "The Weekly Dig" in the free boxes on the street. The annual "Best of Boston" issue of the free Improper Bostonian is always a good bet for finding the kind of establishment that you are in the mood for.
Places densest in bars include:
Canal Street just south of TD BankNorth Garden
Bolyston Street
Landsdowne Street and Fenway area
Harvard Ave/Brighton Ave in Allston
Central Square in Cambridge and Harvard Square in Cambridge
Seaport/Waterfront - specifically Northern Ave, where there are now several popular new bars with roof decks and patios that are packed in good weather
Faneuil Hall
breweries
Samuel Adams Brewery in Jamaica Plain and Harpoon Brewery in South Boston both offer tours and tastings.