<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>         
                <source>1.5</source>
                <target>1.5</target>
            </configuration>
        </plugin>
    </plugins>
</build>

http://today.java.net/pub/a/today/2007/03/01/building-web-applications-with-maven-2.html

Setting any property value for a maven plugin in pom.xml.

This configuration is used to set the compilation version to java5

Setting plugins properties in maven2

Other