Providing good data management services is the hard part of building a database system. When you choose a database system to use in an application you build, making sure it supports the data management services you need is critical. Different database systems offer different data management services. Data management services are entirely independent of the data access services listed above. For example, nothing about relational database theory requires that the system support transactions, but most commercial relational systems do.
Concurrency means that multiple users can operate on the database at the same time.
Transactions permit users to make multiple changes appear at once. For example, a transfer of funds between bank accounts needs to be a transaction because the balance in one account is reduced and the balance in the other increases. Obviously, both the customer and the bank are best served if both operations happen at the same instant. Transactions have well-defined properties in database systems. They are atomic , so that the changes happen all at once or not at all.