One of the challenges of creating a DBMS is managing who can access and change the data.
For example, with personalised websites that require a login, you will only see data that is relevant to you. However, if an employee of that website company accesses the DBMS, they will be able to view all customer accounts. The customer details come from the same database, but different levels of access are given to different users.
A DBMS developer would think about the needs of the user and develop a bespoke system to suit the needs of the company.
With an online music library, there would be different access levels for different users. For example:
Websites use database applications to customise the user interface to suit the needs of each user. The owner of a blog will be able to delete, add and edit comments, whereas the reader would simply be able to view the blog posts.
With social networks you customise who can access your data:
The data that appears on Sarah's profile on a social network is stored in a database. Sarah has set up access rights to her data. Sarah only allows users known as her 'friends' to view her profile. This means that the public and 'friends of friends' cannot see her data.