Time to think about math. Remember sets and the union and intersection of sets? When describing the union or intersection of sets, you described attributes as being in set A AND in set B (intersection of the sets), or in set A OR in set B (union of the sets). Introducing "AND" and "OR" into keyword searches has the effect of limiting or expanding the search results. These are referred to as Boolean (after Mathematician, Boole) operators. "AND" limits a search and "OR" expands it. If you ask for all documents which contain the word "ocelot" AND the word "endangered", you will get fewer pages than if you just asked for "ocelot". Conversely, if you ask for all the documents which contain the word "ocelot" OR the word "endangered", you get all the pages that contain "ocelot" and all the pages that contain "endangered", regardless if the other word is present. Try it now. Try it with different engines.

AltaVista (http://www.altavista.com/)	HotBot (http://www.hotbot.com)
Lycos (http://www.lycos.com/)	Excite (http://www.excite.com)