What is the difference between a CSS and XPath selector? And which is better with respect to performance for cross-browser testing?

I am working with the Selenium WebDriver 2.25.0 on a multilingual web application and mainly test the page content (for different languages, like Arabic, English, Russian, and so on).

For my application, which is better according to performance and make sure that it should be supported for all the browsers (i.e., Internet Explorer 7, 8, 9, Firefox, Chrome, etc.)?