Words such as AND, OR, and NOT are used to combine search terms to broaden or narrow a search in an electronic database.
As you begin your research project take a moment and think about how to keep careful records of where you have searched (what catalog or database) and with what keywords. The system needs to be flexible and dynamic since your project may change focus and you need to adapt. What you want to avoid is repeating work (since you may not remember doing a search one month later) or leaving a hole in your research (e.g., by searching a database or site early on with one idea and then never returning after you have changed directions). You also need good recordkeeping from the start in order to keep track of your citations.
You may want to use a tool to manage your citations and save your sources as you research, both RefWorks and Zotero can help you do this. But you will need to set these up before you start your research, though they both will save you time in the end and help you organize a larger research project.