A scroll compressor has two interleaved spiral-shaped scrolls. One is fixed; the other orbits without rotating, trapping pockets of vapour at the outer edge and pushing them towards the centre, where they leave at high pressure. Several pockets are being compressed at once, so the flow is continuous rather than pulsing.
Scrolls have few moving parts and no suction or discharge reed valves, which makes them efficient, quiet and more tolerant of small amounts of liquid. They are common in larger air conditioning, condensing units for cold rooms and big multi-door or remote systems, but less common in small plug-in reach-ins and chest freezers, where reciprocating and rotary types are cheaper at low capacity.
One practical point: a three-phase scroll compressor must run in the correct direction. Swapped phases can make it run backwards, noisy and not pumping. A good electrician checks phase sequence at commissioning, particularly when a kitchen is rewired or equipment is shifted to a new site.