Searches are made of of three main ingredients: the things you type in, directions on how to search for them, and the rules about the things you are searching.
The things you type in are keywords. Those keywords are then combined according to the default directions (known as boolean operators) to be searched for within each item within the database. You can also provide a set of rules, or limiters, to tell the database what items to search.
The result list is the product of the ingredients you gave the database.