Most databases allow search expressions to be grouped or nested using parentheses. This allows you to be more precise in your search by dictating the order that you want the terms and operators searched. Just as in your College Algebra class, the Boolean Operation which is in a parenthesis is done first.
Generally, databases search the expression located in the innermost set of parentheses first, and then continues the search, moving outward to the terms at the edges of the expression.
Louisiana AND (crayfish OR crawfish)
bin lad?n AND (al-quaeda OR al-qa'ida) AND (money OR financ*)
These searches may yield much different results if the parentheses are omitted. In the first example, Louisiana AND (crayfish OR crawfish) will yield records that deal with Louisiana crayfish or Louisiana crawfish. Louisiana AND crayfish OR crawfish (with parenthesis omitted) will yield records that deal with Louisiana crayfish or deal with crawfish (with or without Louisiana).