DirectShow (formerly ActiveMovie) is a multimedia architecture developed by Microsoft. It first shipped in 1996 and has been part of every version of Windows since Windows 98. It is also available for Windows Mobile.
DirectShow divides the processing of multimedia tasks such as video playback into a set of steps known as filters. Filters have a number of input and output pins which connect them together. The generic design of the connection mechanism means that filters can be connected in many different ways to achieve different tasks, and developers can add their own effects or other filters at any stage in the graph. DirectShow filter graphs are widely used in video playback (in which the filters will provide steps such as file parsing, video and audio de-multiplexing, decompressing and rendering) as well as being used for video and audio recording and editing. Interactive tasks such as DVD navigation are also successfully based on DirectShow.
To get started on developing a DirectShow filter, try the filter wizard for Visual Studio. This will create a Visual Studio project for your filter.