Chaps are leather "overtrousers" commonly worn by riders. Half chaps cover only the lower leg. Full chaps cover the full leg.
Western chaps tend to be looser and flanged. English chaps are almost always half chaps and are snug to the leg. They serve a slightly different purpose.
Western chaps protect the legs and pants from brush and the like when riding in rough country. English chaps are generally worn as a cheaper alternative to tall boots and help protect the lower leg from rubbing but also are often designed to increase grip.
This 1800s cowboy is wearing "shotgun" style chaps. Image from the John C. H. Grabill Collection, public domain, via Wikimedia Commons.