Selenium is a test tool for web applications. Selenium tests run directly in a browser, just as real users do. And they run in Internet Explorer, Mozilla and Firefox on Windows, Linux, and Macintosh. No other test tool covers such a wide array of platforms.

• Browser compatibility testing. Test your application to see if it works correctly on different browsers and operating systems. The same script can run on any Selenium platform.
• System functional testing. Create regression tests to verify application functionality and user acceptance.

Try it out! Get started with Selenium IDE for your first taste of Selenium’s power. You can run Selenium IDE tests in any supported browser using Selenium Core.

Any Language! Want to write tests in your favorite programming language? Try Selenium Remote Control; it currently supports writing tests in Java, .NET, Perl, Python and Ruby.

Supported Platforms:

Windows:

• Internet Explorer 6.0
• Firefox 0.8 to 1.5
• Mozilla Suite 1.6+, 1.7+
• Seamonkey 1.0
• Opera 8

Mac OS X:

• Safari 1.3+
• Firefox 0.8 to 1.5
• Camino 1.0a1
• Mozilla Suite 1.6+, 1.7+
• Seamonkey 1.0

Linux:

• Firefox 0.8 to 1.5
• Mozilla Suite 1.6+, 1.7+
• Konqueror

Selenium uses JavaScript and Iframes to embed a test automation engine in your browser. This technique should work with any JavaScript-enabled browser. Because different browsers handle JavaScript somewhat differently, usually they have to tweak the engine to support a wide range of browsers on Windows, Mac OS X and Linux.

You can read more here.