For example, the results could yield a page which mentions Mark Shelby went to the County Fair in Indiana. OR Finds documents containing at least one of your search words. Shelby OR County OR Indiana finds documents containing any of those words, but not necessarily all of them. This is an under-utilized search term. AND NOT Excludes documents containing specified words or phrases.
Search engines work the same way.
The keywords within the parentheses are searched prior to the application of other search terms. The results will be ranked by order of relevance to your search terms. This control allows you to formulate a search, then tell Altavista what order you want the results ranked. As an example, I wanted to know if there was a site containing Revolutionary War pensions from the state of Virginia. The top ten results took me to a collection of Virginia Revolutionary War Bounty Warrants and Revolutionary War Public Service Claims, and a page of Revolutionary War Pensioners of Scott County, Virginia.