There are lots of different strains and Kush is a popular group of them.
Basically, when Nixon started the war on drugs and the Feds started to really crack down on pot smuggling, two things happened: Colombian drug smugglers switched from smuggling pot to cocaine, thereby creating the great Cocaine boom of the 70's and 80's, and American hippies and rednecks (and hippie rednecks) started to grow their own weed. This was also the time when the Organic Food movement really took off, leading to many hippies becoming experts in traditional horticultural techniques. Both the weed growing and the organic food growing were centered in the Pacific Northwest. Pot, naturally, benefited from this new found hippie expertize in horticulture.
Weed is very easy to hybridize by grafting different strains onto each other. So, for the last ~35 years hippies have been using hybridization techniques to make stronger and stronger strains of weed. The Kush branch is a family of those strains.