Cellulose is an important component of the cell walls of green plants and it's the most abundant natural polymer (big molecule from repeated subunits) on Earth. If the polymer structure is not too big (less than 200 repeated units), cellulose becomes microcrystalline cellulose.
In skin care, it comes as a fine or less fine white powder. The less fine (bigger particle size) version is used as a gentle scrubbing agent (a nice natural, biodegradable alternative to now banned plastic polyethylene) and the ultra fine version is used as a helper ingredient that gives a super-silky, soft touch, reduces tackiness or greasiness and can also be used as a mattifying agent.
Combined with Cellulose Gum, it also works as a nice gelling agent.