A collection of data organized according to strict rules. Creating databases and managing them falls within the realm of web programming and is necessary in cases where website visitors are offered to choose something (product, service, information) from the general pool. In general, a database is the optimal system for storing any information on websites above a certain level of complexity.
Product catalogs, user registration data, archives of guestbook and forum messages - a convenient repository for any information that may be needed by users or website developers.