A skier is a person who skis: they slide over snow on two long thin boards, down a hill or across country. Winter sports, holiday talk, and accident reports all use it. It is a plain noun, built like teacher or swimmer: the thing you do, plus -er for the person. No special formality. The first part is ski, with the “ee” sound, so skier sounds like “skee-er,” not like sky. Though actually, English has look-alikes built from sky, but they are not this word. This skier belongs to snow. One skier, two skiers; the sport or the action is skiing.