The test section in the suite file contains a name for the <test> set, a list of parameters (which can also be declared at the suite level), classes, or packages to run. Both classes and packages can take a wildcard in the entry to run all the test classes in a specific folder or all of them in a package. Here are a couple of examples:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd">

<suite name="My_Test_Suite" preserve-order="true" parallel="false" 
thread-count="1" verbose="2">

    <!--  tests -->
    <test name="My Test">
        <!-- test parameters -->
        <parameter name="browser" value="chrome" />
        <parameter name="platform" value="Windows 10" />
        <parameter name="environment" value="local" />

        <classes>
            <class name="my.tests.RockBandsTest" />
        </classes>
    </test>
</suite>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd">

<suite name="My_Test_Suite" preserve-order="true" parallel="false" 
thread-count="1" verbose="2">

    <!--  tests -->
    <test name="My Test">
        <!-- test parameters -->
        <parameter name="browser" value="chrome" />
        <parameter name="platform" value="Windows 10" />
        <parameter name="environment" value="local" />

        <packages>
            <package name="my.tests.*" />
        </packages>
    </test>
</suite>