Conversion testing is to verify that one data format can be converted into another data format so that the converted data format can be used seamlessly by the application under test appropriately.
Any kind of data can be converted from any form to another; however, for web-based application, the webpages should be of the form HTML that should agree to W3C HTML specification so that browsers can render the page correctly.
Programming language Conversion
Database file conversion
Media conversion(audio, video, image, documents)