What is the difference between a scanner and a receiver?
A scanner is a specialized receiver with features for effectively monitoring many frequencies, classically by tuning to each in rapid succession (scanning) and stopping when a signal is found, then continuing when the signal ends. The scan function can of course be disabled to use it as a receiver for a single channel.
Scanners as commonly discussed are typically used to monitor the communications of public safety services (police, fire department, etc.) because those organizations use many two-way radios on many channels transmitting intermittently. Modern scanners may include the ability to monitor “trunked” radio systems, which have a centralized control channel containing frequency allocations which the individual users' radios automatically follow; this is a feature which is generally unique to scanners as opposed to other types of receivers.
However, scanning is also an incidental feature on many digitally controlled receivers or transceivers — not what they're sold for, but a convenient extra.
On amateur radio transceivers, scanning is useful to efficiently monitor many memorized local repeaters or simplex frequencies, or to continuously sweep across a HF band rather than having to spin the big knob by hand.
Car stereos have scan functions, which you can use when you're out of your usual area to find stations. In addition to an actual scan button, there's probably a function to find the next adjacent station, skipping over dead channels — this is also based on scanning.