Your data would be stored on a database server that can be accessed from any computer inside or outside your own network, depending on how the database is built. Only secure servers with data encryption and security certificates are used to prevent anyone hijacking your data, and access to your database is controlled by passwords. Using the passwords, you can give different individuals different levels of access to the data. For example, you could give a customer access to his own data, each area sales representative access to data for their own customers and give the sales manager access to all customers. You could also control access depending on where the database was accessed from.
When combined, these features make up formidable protection for your data whilst preserving the accessibility and functionality.