Apart from the morning coats that British people still wear to the better sort of daytime weddings, "formalwear" (involving long jackets and white bow ties) doesn't exist any more - it's important to understand that the dinner jacket (AKA tuxedo, or "black tie" dressing) was invented to be worn for private dinners given at home.
As such a dinner jacket is meant to feel relaxed, so that you can party wearing one, although the minimal detailing does allude to its relative formality - traditionally they have neither pocket flaps nor vents on the jacket, nor turn-ups on the trousers. In its classic style, a black or midnight-blue suit, the jacket can take many forms (single or double-breasted, with a peak, notch or a shawl collar with silk facings, and two or three piece) while the trousers are fairly unwavering - they should have a single stripe of silk (grosgrain or satin, to match the facing on the jacket's lapels) running down the outside of the legs. The bow tie is black silk (again to match the lapel facings), the shirt is white and has studs rather than buttons, the socks are black, and so are the shoes, which should be highly polished, or patent leather, Oxfords. A cummerbund is an optional extra, which has somewhat fallen out of favour lately.
However, these days a broad range of options counts as formalwear. The obvious variant is simply a black suit, worn with a white shirt and black necktie - as frequently sported by young Hollywood actors. However, if the occasion allows for a degree of personal expression a more stylish option is to swap the tuxedo jacket for a velvet jacket (which originated from the Victorian smoking jacket), and the leather shoes for velvet slippers.
Learn more about the tuxedo