A suit jacket always has matching pieces: trousers and a vest for men, or a skirt, dress, and pants for women. While I often recommend that women split up a suit to get the most wear from the different pieces (but dry-clean them together!), that advice does not apply to men.
A suit is the most formal style of clothing of the ones described here. For men, that means the pieces should always be worn together. A suit will be the most tailored, with the jacket fitting more snugly than a sport coat or blazer. The weave is typically more subtle, the fabric typically a fine wool with a slight sheen, and the colors and patterns usually somber.