Golf shirts, also known as a polo shirt, is a semi-casual shirt that meets the golf etiquette requirement that a shirt with a collar be worn when playing on a golf course.
Over the years, golf shirts became an acceptable business casual, dress code approved outfit for work use. While tee shirts are cheaper and sometimes more comfortable, the collar gives the golf shirt an air of respectability.
Golf shirts can be tailored to properly fit women, but most of the time they're forced to wear the boxy versions for men.
A popular use for golf shirts is to use them for advertisement. Usually a logo or verbage is embroidered over the left breast. I personally have several golf shirts custom embroidered with my name (useful for the conventions I attend), and I have polos from my publishers. Golf shirts with tee shirt styled printing on them are not very common, but custom ones can be created.