Database design ensures that the information you put into your web application is stored in a manner that allows you to retrieve it the way you want it. Starting with the information developed during web application design (knowing what is going into the web application and what must come out), database design defines how the information is stored.
A database is composed of tables (think columns and rows like a spreadsheet) and relationships between those tables. During this step, a design for all the tables is defined. The type of data held in each table and the relationships between tables are also determined. Pre-populated tables are defined and the information that needs to be entered in them is stated, for example, a table to hold the names of the 50 states of the United States. The end result is a created database structure and a data dictionary defining all the information held within it.