The "Content-Type" is an HTTP header used to indicate the media type of the resource being sent or received. It helps the client and server understand how to process the data. For example, a Content-Type of "text/html" tells the browser to render the content as a web page, while "application/json" indicates that the data is in JSON format, which is often used for APIs. Different media types are defined by the IANA (Internet Assigned Numbers Authority) and can include types like "image/jpeg" for JPEG images or "application/xml" for XML data. Specifying the correct Content-Type is essential for proper data handling and rendering in web applications.