within mysql you have databases or just one database. Within that data bases you can have tables. Those tables contain data. The cool thing about tables is that they can relate to other tables within the database. For instance: instead of putting all of the customers, books for sale, orders, mailing list in one table you can put them in multiple tables and make them all related. It is much quicker and easier to pull something up like: How many books did mary buy and what books did she buy and how much did she spend on all of those books.
