The simple definition of a tourist is someone who is traveling, usually for pleasure.
Tourists are the majority of travelers, when not vacationing. Tourists may be couples, families, or just a person or two who visit locations and like to take in history, culture, local food, etc.
Tourists are sometimes negatively stereotyped to be more naive, purchasing expensive guidebooks, visiting the most famous of attractions, and perhaps suckered into purchasing souvenirs at a high price and with no relevance to the place (t-shirts, key chains, etc.).