A scale is one of the two handle slabs fastened to either side of a knife tang or liners. Everything you actually grip is scale, and the material choice affects grip, weight, durability and price more than almost any other handle decision.
G10 and Micarta are laminates and dominate modern working knives. Aluminum and titanium are metals, durable and cold to hold. Bone, stag and wood are traditional naturals. Two of those meet in bone or wood. FRN is molded polymer, light and cheap.
Scales attach with screws, pins or rivets. Screwed scales can be removed for cleaning or replaced with aftermarket ones; pinned scales are permanent.
Grip when wet is the thing most people underweight. Smooth polished scales look excellent and become slippery with water or oil. Textured G10 stays grippy in every condition and is harder on your pocket lining.
Natural materials move with humidity. That is why traditional bone and wood handles occasionally sit slightly proud of the bolsters after a dry winter.