Look around. Every contained space has some kind of an Interior Design: homes, offices, hospitals, stores, offices, hotels, resorts, theaters, restaurants, and everything else with four walls. Some of it is tasteful and some of it hideous, but it is Interior Design nonetheless.
As a college major, Interior Design is heavily pre-professional and very hands-on (expect a lot of projects that simulate on-the-job situations). It falls into the broader category of design. If you decide to major in Interior Design, you’ll study light, color, form, space, furnishings, and pretty much all other aspects of built environments. You’ll study the basics of three-dimensional design, space planning, and perspective. You’ll also study color theory, different kinds of materials and patterns, and computer-aided design, which is all the rage these days.
Once you get into the major a bit, you’ll probably have the choice of specializing in residential design or commercial design.