On the Internet and as defined by Request for Comments ( Request for Comments ) 1521, a media type (also referred to as a content type ) is a general category of data content, such as: application (executable program), audio content, an image, a text message, a video stream, and so forth. The media type tells the application that receives the message what kind of application is needed to process the content (for example, RealAudio or a similar program to play the audio content for a user).
Each of these media types have subtypes. For example, the ;"text" media type has four subtypes: plain, richtext, enriched, and tab-separated values.
E-mail users sometimes receive an attachment that describes its media type and subtype. For example, the media type and subtype of: "application/octet-stream" describes an application organized in eight-bit units ( octet ). (Since this content is likely to be an executable program, it should be saved to disk so that it can be scanned by virus-scanning software before trying to run it.) Your browser may have assigned an application to handle each of several media types and subtypes or it may ask you to select an application. In the latter case, you need to know one or more applications that handle that particular media type and subtype.