Tourism is generally regarded as the act of traveling to a different location, for either business or pleasure purposes. However, it actually has quite a specific definition: the act of traveling to another environment, for at least 24 hours, but for no longer than one year, for purposes related to business or leisure.
A tourist is generally only classed as such if they stay in overnight accommodation situated in the location they travel to. By its very nature, tourism involves a round trip, rather than travel in one direction only. With this in mind, the tourism industry refers to all aspects of the service industry that cater to tourists.
Travel refers to the act of moving from one location to another. This can refer to long-distance travel, short-distance travel, overseas travel, domestic travel, and various other forms. Crucially, travel also includes both round trips and one-way journeys, and it covers a wide variety of different travel purposes.
The travel industry, therefore, refers to the numerous aspects of the wider service industry which cater to the needs and desires of those who have traveled from one part of the world to another.