When you have a research topic, you need to break your question into its key concepts. For example, if you are researching the case surrounding Jessica Colotl and the possible deportation of other illegal immigrant students, some of the key concepts are:
Then when looking for articles, you need to think of how someone else would word the same idea. Think of like terms. Use the correct trucation symbol to get multiple endings. In most databases, the "*" is the truncation symbol (you can always check the help screens in the database if "*" doesn't work). So, if you type immigra*, you will get immigration, immigrant, immigrants...
Some possible search terms for the above research would be:
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.
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.