AND: The default operator. If you add no character (or an "&") to the beginning of the search term, this is the operator it will use. If a term is using AND, a result must match it or else the result won't show up.
INCLUDES: If you add a "+" character to the beginning of the search term, this is the operator it will use. If a term is using INCLUDES, it will remove all results that don't match it.
NOT: If you add a "-" character to the beginning of the search term, this is the operator it will use. If a term is using NOT, it will remove all results that do match it.