In short, the black tie dress code features a dinner jacket (or a tuxedo, as it’s known in the US), and matching trousers. The look is completed with a pleated white shirt, black dress shoes and a bow tie (usually in black, though this is less strictly adhered to today). There’s also the option of a cummerbund or waistcoat to really set your outfit off.
As the second most formal of all dress codes (just behind the uber-strict white tie code), black tie comes with a pretty tight set of dress rules which must be obeyed.
Over the years, the rules of black tie have relaxed slightly, however we’d usually recommend sticking to the classic selections, while allowing the little details to subtly set you apart from the crowd. As Gay Talese points out above, being, “a man to whom the details matter,” can make a telling difference.