Author Searches try to match what you type in the author field. An author can be a person or an organization.

Title Searches try to match what you type in the title field.

Subject Searches try to match what you type in the subject heading field. Subjects can be people, places or things.

Keyword Searches try to match items in all of these fields.
It is the broadest kind of search and will give you the largest list of results.