AND => you both both terms or phrases in your search result(s) ... Example: zombies AND robots BOTH have "no emotions" >> Searching "no emotions" would get you information on robots AND zombies.
OR => you'll take either or both terms or phrases in your search result(s) ... Example: You could say that aliens OR zombies could be dangerous because they have a "taste for flesh" >> Searching either aliens OR zombies would bring in either of those subjects, as well as things they have in common ("taste for flesh").
NOT => you don't want this word or phrase in any of your search results (NOTE: this could excude relevant material just for mentioning that word or phrase) ... Example: SPACESHIPS is not on the diagram below; a NOT would fall outside the diagram.