When you first start a database search engine, it provides you with a basic form, usually with only one box in which to enter your search terms.  For example, the WebLUIS Catalog provides a box to choose the type of search followed by a single box in which to enter search terms:

If you have a broad, unfocussed search, this makes a quick search easy. But notice on the top and bottom gray bars there is also an option for an advanced search. (Some search engines call this "guided search".) This usually allows for more complex searching, sometimes including limiting to formats or other desired qualities. (Notice on the WebLUIS advanced screen, show below, you may limit to locations, language, or format, as well as use multiple entries.)

Also note the radio boxes (highlighted in pink, above, with "and", "or", and "not") between the entry boxes. These Boolean connectors will be discussed next.