Practice centers are places in a larger database, typically a legal research database, that organize resources for particular kind of practitioner. One of the best examples is the tax practice center on Bloomberg. By organizing the page this way, the databases create a one stop shop for certain kinds of practitioners. If you are researching a certain topic, finding a practice center page can be invaluable because it will provide primary and secondary material. In addition, it will usually tell you the standard treatises and practices tools used by practitioners in that area.