Interests are stable preferences for certain activities, environments, or types of work over others, and are one of the most practical individual-difference constructs for career decisions.
Interests are related to but distinct from personality traits. Personality describes how you tend to think, feel, and behave; interests describe what you're drawn to spend your time doing. Two equally extraverted people might have very different interests — one drawn to sales, the other to teaching — even though both roles reward sociability. That's part of why interests are often assessed alongside, rather than instead of, personality traits when the goal is career guidance.
The most influential framework here is Holland's RIASEC model, which sorts interests into six broad types: Realistic, Investigative, Artistic, Social, Enterprising, and Conventional. Most people aren't a pure type but a combination of two or three, and the model has held up reasonably well across decades of vocational research, which is more than can be said for most pop-career-quiz frameworks.
Interests tend to be somewhat more responsive to experience and environment than core personality traits, which is consistent with everyday intuition — you can develop a new interest through exposure far more easily than you can become a fundamentally different kind of person.