This chapter of the guide introduces the basic concepts of &GStreamer;. Understanding these concepts will be important in reading any of the rest of this guide, all of them assume understanding of these basic concepts. An element is the most important class of objects in &GStreamer;. You will usually create a chain of elements linked together and let data flow through this chain of elements. An element has one specific function, which can be the reading of data from a file, decoding of this data or outputting this data to your sound card (or anything else). By chaining together several such elements, you create a pipeline that can do a specific task, for example media playback or capture. &GStreamer; ships with a large collection of elements by default, making the development of a large variety of media applications possible. If needed, you can also write new elements. That topic is explained in great deal in the &GstPWG;. Pads are element's input and output, where you can connect other elements. They are used to negotiate links and data flow between elements in &GStreamer;. A pad can be viewed as a plug or port on an element where links may be made with other elements, and through which data can flow to or from those elements. Pads have specific data handling capabilities: A pad can restrict the type of data that flows through it. Links are only allowed between two pads when the allowed data types of the two pads are compatible. Data types are negotiated between pads using a process called caps negotiation. Data types are described as a GstCaps. An analogy may be helpful here. A pad is similar to a plug or jack on a physical device. Consider, for example, a home theater system consisting of an amplifier, a DVD player, and a (silent) video projector. Linking the DVD player to the amplifier is allowed because both devices have audio jacks, and linking the projector to the DVD player is allowed because both devices have compatible video jacks. Links between the projector and the amplifier may not be made because the projector and amplifier have different types of jacks. Pads in &GStreamer; serve the same purpose as the jacks in the home theater system. For the most part, all data in &GStreamer; flows one way through a link between elements. Data flows out of one element through one or more source pads, and elements accept incoming data through one or more sink pads. Source and sink elements have only source and sink pads, respectively. Data usually means buffers (described by the GstBuffer object) and events (described by the GstEvent object). A bin is a container for a collection of elements. A pipeline is a special subtype of a bin that allows execution of all of its contained child elements. Since bins are subclasses of elements themselves, you can mostly control a bin as if it were an element, thereby abstracting away a lot of complexity for your application. You can, for example change state on all elements in a bin by changing the state of that bin itself. Bins also forward bus messages from their contained children (such as error messages, tag messages or EOS messages). A pipeline is a top-level bin. As you set it to PAUSED or PLAYING state, data flow will start and media processing will take place. Once started, pipelines will run in a separate thread until you stop them or the end of the data stream is reached.