ELI5: What is difference between grid computing and computer cluster

I'm reading about supercomputers on wikipedia and I read that supercomputers usually work like grid or computer cluster. I dont understand difference, if we have grid network of computers it means that they are all connected with each other and they work together(but in grid computers work more independently and use computers only if they are available). But if we have cluster computers, they still need to be connected, if applications isnt hard enough on demanding power you are still not going to use all power :S I'm really confused about these two, somewhere I read that grid is for whole PC, but then I go to another page and I read that grid also can mean that you for example can have grid network of CPU or GPU :S And same thing goes for computer cluster... More I read about them more Im confused what is what :'(