There are three types of searches, with 8 search fields available:
If a search uses multiple fields, they are joined by AND in the search. Example: Author: Darwin AND Title: Species. In other words, search results must match both criteria.
Within each field of a Keywords search, multiple words are, by default, joined by AND (i.e., the "all" button to the right of the field). Example: Title: Applied Physics is treated as a search for articles with titles having both Applied AND Physics (not necessarily next to each other). Use the "any" button next to the search fields to change this to OR (Applied OR Physics), or use the "phrase" button to change this to an exact phrase ("Applied Physics"). Boolean operators may also be used within the Keywords fields.
Help for each of these fields is available under Main Search Strategies.