A Group Of Pictures (GOP) consists of all the pictures that follow a GOP header before another GOP header.
The GOP layer allows random access because the first picture after the GOP header is an Intra picture that means that it doesn't need any reference to any other picture.
The GOP layer is optional, i.e. it's not mandatory to put any GOP header in the bitstream.
In the header there is also the timecode of the first picture of the GOP to be displayed.

The decoding process, as the GOP header is immediately followed by an Intra picture, can begin at that point of the bitstream. Anyway it's possible that some B pictures, following such I_picture in the bitstream, have references coming from the previous GOP and can't be correctly decoded.
In this case the GOP is called an Open GOP because some references from the previous GOP exist; if a random access to such a GOP is performed, some B_pictures shouldn't be displayed .
A GOP is called a Closed GOP when either there are no B_pictures immediately following the first I_picture or such B_pictures haven't any references coming from the previous GOP (in this case a GOP header flag must be set).

In the "coding people" language the GOP length is the period (often expressed in frames) by which an Intra frame occurs. It must be noticed that such a value cannot be found in the bitstream and it is unnecessary to the decoding process. Furthermore it isn't specified any fixed period for the Intra frame. As the presence of the Intra frames is quite important for many applications, it is the encoder that has to provide them, while the decoder has only to work with all the valid bitstreams.