In the USA, Disneyland is the name of Disney's property in Anaheim, California.
Originally, Disneyland was a single theme park, and still is. It has various versions around the world, but I imagine here we're talking about the Disneyland theme park in California.
Confusingly, since opening, Disney has also opened another theme park (Disney's California Adventure), a shopping area, and hotels around the original Disneyland park. The overall resort here is called the Disneyland Resort.
Walt Disney World, on the other hand, is Disney's resort in Florida. This is far, far larger than the Disneyland Resort and contains far more - more parks, hotels, shopping, and areas to expand into.
Walt Disney World contains the Magic Kingdom, Florida's version of the Disneyland theme park. However, in my experience, a lot of people call this park "Disney World", which is where I believe a lot of confusion stems from.