Lorelet here - what's the difference between a chapter and a legion?

I thought all the primarchs were leaders of legions - so Russ is leader of the Space Wolves legion, which is like 20k marines. Then there are 20 chapters within (1K marines). However, I see there are is a chapter master for the "Space Wolves Chapter", Logan Grimnar. So are the space wolves a chapter or a legion? Who is the legion then?