from my experience downhill is where you are lifted to the top of a mountain and you do exactly what is in the name. you go downhill. freeriding is like downhill but can be done on any mountain any where with any bike. again to me it means whats in the its a free ride, or in other words riding without purpose for fun.
hope i helped
You will find that DH bikes will have slacker headtube angles and shorter stays which offer more stability at speed. They will also have lower bottom bracket heights to keep the weight low on the bike.
FR bikes are usually also overbuilt and heavy to gontend with their huckable nature where-as purebred DH race bikes are as light as possible. TFR bikes will feature steeper head tube angles (better low-speed manouverability and seated pedalling position), highr BB (to clear obstacles), shorter wheelbase, stays and top tube (better tight manouverability). As you can see, this is very general and there is alot of overlap but yeah. Hope that helps.