These days, it's semantics, really. Especially in the United States, a "blazer" has come to mean any button up jacket with lapels that is worn with pants of a different fabric. But a blazer traditionally indicates a navy jacket, usually with patch pockets (and possibly a patch on the chest denoting a yacht club) fastened with metal buttons. A sport coat, historically, was more of a rustic, hunting jacket. Its pockets usually have flaps and there may be, as of late, a small ticket pocket situated above the normal pockets. They are typically cut from heartier fabric and have a softer shape than a suit jacket. Which, it should be stated, is the most refined of them all—made from lighter and smoother worsted wools, cottons and linens.