A blazer is a type of jacket with large metal buttons that usually comes in navy blue. While blazers used to be part of sport dress for yacht or cricket, they are not the same thing as a sports coat.
Original blazers were single-breasted jackets with colorful stripes that had school or club emblems. There are many stories about where the term came from. One of the stories is that the name came from a rowing club in Cambridge, who were red jackets which looked like a blazing fire on the river. Another story says that a ship called the HMS Blazer created new uniforms that were navy jackets with brass buttons. Blazers became an important part of school outfits for boys, especially at schools like Eton. Blazers also became popular in America, but they were less formal.
Contemporary blazers are double-breasted. There are two ways to wear blazers. One is the more formal way, with trousers, and a shirt and tie. A less formal way to wear a blazer is with chinos or jeans.
It may seem like all jackets are the same, but there are big differences between a blazer, a suit jacket, and a sport jacket.
A sport jacket has nothing to do with tracksuits or t-shirts. The term sport jacket came about because gentlemen used to do sports like horse riding and hunting while wearing them. A sport coat is less formal that a suit coat.
Sport coats are usually made of wool, tweed, or more exotic fabrics like corduroy, cashmere, or velvet. It usually has two to three buttons and can have a texture or pattern.
The cut is similar to a suit jacket. An American style jacket is has one cut in the back while a British style jacket has two cuts and a continental style jacket does not have a cut.
A sport jacket that is matched with nice pants and a shirt is a good for business casual events. For more informal occasions, a solid navy jacket can be paired with jeans or chinos and a colorful shirt. Sports jackets are not appropriate for formal meetings and parties.