Microsoft SQL Server JDBC Driver version 2.0

Microsoft SQL Server JDBC Driver version 2.0 supports both JDBC 3.0 and JDBC 4.0. To support backward compatibility and possible upgrade scenarios, the JDBC Driver version 2.0 includes two JAR class libraries in the installation package: sqljdbc.jar and sqljdbc4.jar.

The sqljdbc.jar class library provides support for JDBC 3.0 and requires a Java Runtime Environment (JRE) of version 5.0. The sqljdbc4.jar class library provides support for JDBC 4.0 and requires a Java Runtime Environment (JRE) of version 6.0 or later. The sqljdbc4.jar includes all of the features of the sqljdbc.jar as well as the new JDBC 4.0 methods.
SQL Server JDBC FAQ