A database is a place to store your application data. It can be a simple text file, xml file, binary file, MS Access database, SQL Server database, Oracle database etc. In other words, your database need not be SQL Server or Oracle - it can be just plain text files.
However, there are several key differences between storing data in text files and database server based databases. Text file based database storage may be good enough for small applications which deal with very less amount of data. Most of the applications developers followed this approach before database systems like FoxPro, MS Access, SQL Server etc became popular.
Data storage and retrieval become very efficient if you store very large volume of data in text files.
The popular database systems in the market are SQL Server and Oracle along with few others. In the coming chapters, you will learn how the database systems evolved and how to use them.