The term “blazer” is generally used to refer to a jacket made out of a fairly smooth, relatively lightweight cloth; this kind of jacket is thought to have originated in the Navy, and consequently you can still find blazers with brightly polished buttons and a badge on the chest pocket, although blazers have moved far away from this nautical theme. Blazers are often thought of as summer wear, and navy blues and brighter summer colours abound.
Sports jackets, on the other hand, are made out of heavier, tweedy materials that are suitable for autumn and winter wear; browns and greys are frequently found, and checks and herringbones are often the order of the day. Not surprisingly given the name, they originated as a loose, comfortable jacket suitable for shooting parties on country house weekends.
Nowadays, both blazers and sports jackets are popular choices when you want to look smart but avoid the businesslike tone of a suit. The beauty of both blazers and sports jackets is that you can wear them with almost any kind of trousers, even jeans. What you wear underneath these jackets is up to you, so you can run the gamut from knitwear to shirt and tie. That makes both blazers and sports jackets extremely versatile, and the smart man will do well to keep more than one of each on hand. Some should be more tailored, others more relaxed. Then you’re ready for all occasions.