what is the difference between search engines that default in AND,OR?

Answer 1

So, let's say you typed in "bob was dead" into the search line.

an "AND" engine would require that all 3 words "bob" "was" and "dead" appear in the page in order to match.

an "OR" engine would show you any page that contained "bob", "was", or "dead".

See?

Answer 2

Just what it says, in accordance with what the search engine does with the entered text

AND - all words together
OR - any of the words