Database systems are a complete index of data put into one location. Databases are used in all businesses weather small mom and pop size to multimillion worldwide companies. They are used to keep track and accurate points of the data that their company has. As well as customer interaction with the company. A customer would use a hotels database to find what hotel rooms are available to rent. A Company uses a database so that all employees that need to access it can access the same one so other departments are not doing things without other departments know. Meaning if HR fires someone and payroll may not know if they do not have access to it, then the fired person would still get paid. But, by using a database system all the department know what each other is doing so when HR fires someone Payroll knows to stop paying that employee.
Each instance of SQL Server has four system databases (master, model, tempdb, and msdb) and one or more user databases. Some organizations have only one user database, containing all the data for their organization. Some organizations have different databases for each group in their organization, and sometimes a database used by a single application. For example, an organization could have one database for sales, one for payroll, one for a document management application, and so on. Sometimes an application uses only one database; other applications may access several databases.