They both seem the same.

CRJ are Canadian Regional Jets made by Bombardier and MD were McDonald Douglas planes (now made by Boing). They're both passenger jets but that is as close as they get to related.

CRJs r made by Bombardier. Mostly in Canada
They're newer, with more efficient & quieter engines. The airframe is lighter, with more composites, & they have greater short field capabilities. They seat less than the MD-80s family.
I've flown on CRJs & MDs & l have both aircraft types flying over my house everyday. When an MD comes over, it is a throaty roar, some picture frames vibrate in my house. The CRJ is engines emit more of a whine, & only when it is right above you, then u get a little roar. My house is not moving on its foundations. l thank bombardier for that.

MD-80 series planes carry more people. The engines r older, less efficient but sounds like they have lots of power available. l find they have a rather long takeoff run. Way more than practical for a plane that size. Also, their landing rolls r significantly longer because of the brake design & also because the thrust reversers r the bucket type. Pilots cannot deploy them until all wheels r on the ground from fear of dragging the buckets on the runway. the CRJs have the kind that just splits along the engine nacelle which can be deployed as soon as the main wheels spin up to speed.

the only similarities would be the rear mounted engines, the gear arrangement & the single aisle.

No relation, other than the fact that they both carry passengers and have rear-mounted engines. They're really two completely different planes, made by different manufacturers, in different countries.