A jogger is a person who jogs: someone who runs slowly and steadily to keep fit, not to win a race. You see joggers in parks and along the road in the early morning, often in soft shoes. The word is ordinary and a little informal. Runner can mean anyone who runs; jogger tells you the pace is easy and the point is exercise. It is simply jog plus -er, a person who does that. Though actually, in British English especially, joggers can also mean the soft trousers people wear for that kind of run (what some people call sweatpants). If someone says they will put their joggers on, they may mean the clothes, not that they are about to go for a run. The person-meaning is still the main one.