Open formats are those file formats whose specifications are published openly and are not restricted to a single vendor or a software or a web application. With the open availability of specifications, the data in the open file formats can be accessed from any software/application that supports the file specification. You are not locked in with a single vendor's software or web application. Examples of open formats include plain ascii text, html, odf, pdf, dvi, rtf, ogg, png, etc..
