Major junior (the 3 CHL leagues, which all vary a bit) is more wide open offensively, less defensive attention to detail, not great gap control, focus on skill & creativity, worse goaltending, higher scoring. All of this is in part due to the younger age range (16-20/21 year olds). The O and the Q aren't physical at all, while the Dub has been more physical, although that has dipped in recent years.
US junior (USHL specifically) is far less skilled, leans a bit older (more 18-19 year olds), focus on defensive structure, but still not as quality from a structural standpoint as the NCAA. Far fewer high scoring games than in the CHL.
NCAA Hockey is tighter checking, less wide open/free flowing, focused on defensive structure, lower scoring, better goaltending. Obvious exceptions in B1G & Hockey East with certain programs, but this is the general assumption.
I'm only saying this with 80% confidence as I haven't watched enough junior in recent years, but this is my general understanding.