For many of your searches, you can employ a more systematic approach to how you search, where you search, and what items you include in your review. Systematized searches tend to be more comprehensive - both in the terms used and the databases used - and with the goal of being transparent about the choices made and providing a reproducible methodology. Even if you are not doing a full systematic review, you can borrow aspects of this methodology to give more structure to your review methodology and have a means for communicating how you found what you have.
So What are We Really Talking About?
While systematic reviews are becoming more common in social sciences, SR are much more common within the medical and health sciences arena. They are often done to address a very specific question or might be done to map the literature to determine where the gaps are within the published literature. For more on the steps of the process, check out the PRISMA checklistor UC San Diego Library's Preparation Checklist.