AND focuses or narrows a search.
If you want to answer the question "What is the impact of advertising on smoking in adolescents?" you can combine all of your keywords with "and."
Smoking and adolescents and advertising will return results will have all three keywords in the article.
OR broadens or expands a search to find all the words connected with "or." Remember that a keyword search only looks for the word you enter. OR lets you put in all possible keywords that might mean the same thing.
teenagers or adolescents finds all the articles that have either word in them.
girls or women or female
cell phone or smart phone
bulimia or eating disorder