What is difference between Borland Silk test and Selenium?

Selenium is completely free test automation tool, while Silk Test is not. Only web applications can be testing using Selenium testing suite. However, Silk Test can be used for testing client server applications. Selenium supports following web browsers: Internet Explorer, Firefox, Safari, Opera or Konqueror on Windows, Mac OS X and Linux. However, Silk Test is limited to Internet Explorer and Firefox.
Silk Test uses 4Test scripting language. However, Selenium test suite has the flexibility to use many languages like Java, .Net, Perl, PHP, Python, and Ruby.