Boston is a seafood city first and foremost -- Massachusetts is the second-largest fishing state after Alaska, and it shows on every menu. The North End is the place for Italian, Chinatown delivers authentic Asian cooking, and the South End has become a hub for creative, upscale dining.
Do not leave without trying proper New England clam chowder. Legal Sea Foods is the reliable chain for it, but you will find excellent bowls at Quincy Market and in waterfront restaurants across the harbor.