I think everyone knows the difference between typing on a typewriter and working with a word processor. Many of the differences between these two tasks are the same differences you find with editing with linear and nonlinear editing systems. Like using a typewriter (typing words in the order they must appear on the finished page), when you are editing in a linear editing system, you need to record from a source tape to an edited master tape each shot as it comes in the program and for the duration it lasts. If you make a change to these shots after you have edited past them, you have to rerecord every shot past that change to move it either later or earlier on your edited master. Linear editing is constrained to this method. If you decide to delete one of the shots (or erase a word), you have to record over it with the next shot and rerecord all the following shots (just as you would do if you wanted to change a sentence on a typewriter).
Nonlinear editing is more like working on a word processor. If you delete a word, all the rest of the words move over next to the last word you typed before the deleted word. You can do the same thing with shots on your computer in a disk-based nonlinear video editing program such as Final Cut Pro. If you want to move any shot or even groups of shots, you can do so without any of the penalties associated with linear editing. No rerecording is involved. You just delete the offending shot and play back the new sequence without it.
How is this accomplished? Nonlinear editors don't rerecord anything until you export the edited sequence from the computer to the output medium of your choice. This is much like printing a document from a word processing program. Nothing is complete until you've printed the document. The same is true of nonlinear editing. You can make changes anywhere in your program until you are ready to show it to the world. You can do this because you are simply asking the computer to play back the shots or parts of shots in the order you have programmed them with your editing software.