Disneyland: original park opened in the 50s. smaller and Disney just bought the land he needed not the land he wanted. Disneyland is the name of the actual park. in the 90s they added Downtown Disney and Disney's California Adventure. they also have a couple of hotels. the entire thing is called the Disneyland Resort. you can walk anywhere on Disneyland Property because its all connected like a huge block on steroids and then hit with a growth ray! :)
Disney World: name for the complete resort build in 1971. originally two hotels and a theme park called the Magic Kingdom. people will call MK Disney World but that is incorrect. for about a decade that was the only park so the name just stuck. currently it is made up of four theme parks, two water parks, over a dozen hotels, Downtown Disney (shopping and dinning area), Pleasure Island (clubs), golf courses and an indoor amusement park (Disney Quest, mostly video games). it is extremely large. you have to drive from one park to another.