Modern database systems will not allow you to directly access the database files. Instead, you must access an application called 'database server' and send commands to the server. This server will understand the commands and perform appropriate action.
In order for the database server to understand the commands, you must use a specific language. This language is called 'Structured Query language' (SQL).
What is the need for a specific language for database access?
There are several database servers exist now (For example, SQL Server, Oracle, Sybase etc). If each of these systems have their own protocols and commands to communicate with them, it will be very hard for programmers to learn each one of them. This is the primary reason a common language was introduced. All database systems are required to understand this common language (SQL).
SQL is an ANSI (defined by American National Standards Institute) standard computer language for accessing and manipulating database systems. All ANSI compliant SQL systems should support the language rules defined by ANSI.
The bottom rule is, if you know SQL, you can work with any modern database systems.
All relational database systems are required to support the standard SQL. In addition to the standard SQL, each of the database systems support additional commands (extensions to SQL). If you are using any extensions provided by SQL Server, it may not work with Oracle and vice versa.