The terms AND, OR and NOT can be used to refine your searches - this very useful technique is often referred to as Boolean searching and can be illustrated with these diagrams:
e.g. "global warming" AND "climate change"
This search using AND decreases your search results.
OR finds either keyword, which expands your search results because
only one keyword needs to be present.
e.g. "global warming" NOT sunspot
In this search you will not find any search results with the word 'sunspot' and this will decrease your search results.