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 where 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.