If you originally purchased a font for print materials, you must purchase web licensing separately in order to use the font on your website. For example, if you use FF Scala for your business cards and want the text on your website to match, you will need to purchase FF Scala Web.
However, if you are embedding a font into an image and using that image as content for your website (i.e. the font is only seen as a graphic representation in a picture or image such as in a banner or advertisement), then you do not have to buy a separate web license under most foundries' eula.
Webfonts are optimized for on-screen use and come in special font formats (EOF and WOFF) to be used specifically in and for web design. Converting an OTF, TTF, or any other font format design for print to work with your website is not allowed.