NOT is used to exclude specific terms from a search. Much like AND, it narrows a search, but unlike AND, it requires that a given term not be present.
Once again, you're hungry and need to do a burger search. This time you want any type of burger with mustard and ketchup, but not pickles. You'd type in your search like this:
A Venn diagram may help to explain:
The entire "pickle" circle would be excluded from the search. So burgers with ketchup and pickles, mustard and pickles, or mustard, ketchup, and pickles would all be excluded.
NOT is probably the least used of the Boolean connectors, but it can be quite useful. Say, for instance, you're interested in war veterans, but not veterans of the Vietnam War. You can use NOT to help you find what you need. You could phrase your search like this:
war veterans NOT vietnam
Some other examples:
apes NOT gorillas
drug abuse NOT marijuana
lung cancer NOT smoking