Databases generally have one of two basic forms - the single-file database or the multi-file relational database. Single-file databases are often called "flat file" systems and relational databases are frequently known as "structured" databases.
The type of database system that you require depends on a number of factors, such as:
If your requirements are simple, eg monitoring the names and addresses of around 100 customers, you might find that standard office tools such as a spreadsheet might be all you need.
However, as your needs become more complicated you will need more a sophisticated and capable database. This can be specifically designed to your needs.