I've heard tell that a lot of sites have had problems with the "obvious" meanings of AND and OR. Turns out that a lot of users imagine that a query for "dogs AND cats" will get them all the documents about dogs, and also all the documents about cats -- that is, they think of AND as meaning what the programmer means by OR. There are sites that try to avoid these terms because of the possibility of confusion, and for all I know there are misguided sites which get them backwards.