Content types description

Anything

Matches any request

Images

Pictures and Graphics

Media

Audio and Videos (only html5 things, videos loaded by plug-ins like flash fall into Object category)

Styles

Cascading Style Sheets (CSS)

Fonts

Scripts

JavaScript files

Objects

Objects handled by plug-ins (flash, java, etc.). Requests from objects for anything also fall into this category.

Frames

Usually, HTML Frames (frame, iframe), any kind of subdocument generally.

XHRs

XMLHttpRequest. Briefly, this is a mechanism JS scripts use to make requests.

WebSockets

This is also used by JS to communicate with a server, unlike XHR, WebSockets are not one-time requests for something, but a persistent connection with a server (usually).

DTDs

Requests for document type definitions.

Pings

Request sent when you click a <a ping="..."> link.

Uncategorized

Anything that does not fall into other categories.