An airliner is a large, commercial aircraft operated by an airline for transporting people and/or cargo.
An airliner is identified mainly by size: few people would call a Cessna Caravan an airliner, even though it's used in scheduled airline services in various parts of the world. (A couple of things many people would call a Caravan would be an air taxi or commuter plane, things that these small aircraft are quite good at.) Airliners may have any type of engine: smaller, shorter-range airliners generally use turboprop-powered (or, occasionally, piston-engine-powered) propellers, while large, fast, long-range airliners almost always use jet-engines (nowadays, this means a turbofan, although the first jetliners used turbojet engines instead), and are called jetliners as a result.
Use this tag for general questions about aircraft used for commercial airline operations, but if the question is about a specific make or model of aircraft, then it's better to use a more specific tag; e.g., boeing or airbus-a320.