When you have a research topic, you need to break your question into its key concepts. For example, if you want to find a case about risk management and the Americans with Disabilities Act, some of the key concepts are:
Americans with Disabilities Act
Then when looking for articles, you need to think of how someone else would word the same idea. Think of like terms:
risk management (I am not sure how else someone might word that)
Americans with Disabilities Act OR ADA OR disabled OR disabilit!
sports OR athlet!
When you have an advanced search screen, you can put each concept in its own line. Make sure the similar terms are separated by OR. Then you connect the different lines with AND.
You can also get fancy and use ! to get multiple endings. So, if I type athlet!, I get athletes, athletics, etc.
Just so you know, nearly every other database we subscribe to uses a "*" for alternate endings. Lexis Nexis is the only one that uses an "!"
A way to understand OR vs AND can be demonstrated by a diagram:
A OR B will get you all information that has either term. So you get the largest amount of hits. In the drawing, this is the entire colored region.
A AND B will get you the part where A and B intersect. In the drawing, this is the green section in the middle.
So, OR broadens your search while AND narrows it. Both are useful for different search functions.