Only web applications can be testing using Selenium testing suite. However, QTP 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, QTP is limited to Internet Explorer on Windows.
QTP uses scripting language implemented on top of VB Script. However, Selenium test suite has the flexibility to use many languages like Java, .Net, Perl, PHP, Python, and Ruby.