Q. What is boolean logic?

Here is a link to a wikipedia article that explains this in (probably) more detail than you want.

http://en.wikipedia.org/wiki/Boolean_algebra_(logic)

In the library/information fields we typically consider Boolean Logic to be analogous to Venn Diagrams, and it is the primary logic used by search engines, and relies on the operators "and" "or" and "not"  Most search engines now assume the "and" operator, which means any terms entered will have to be included in the results.