1. The Heptarchic System
From hept ("seven") and archy ("realms/kingdoms" - from archon: "ruler"), this is, as you can imagine, entirely devoted to the seven "old" planets (the ones that can be seen with the naked eye). This was delivered first and includes the basic tools that were deemed necessary for the later Enochian work - such as the Sigillum Dei Aemeth, Lamen, Table of Practice, Seals of the Bonorum, etc. There is a lot of material here (most of which can be found in the Five Books of Mystery [Quinti Libri Mysteriorum] if anyone wants to check the source works), the majority of which isn't used by modern magicians, barring the Sigillum Dei Aemeth, as most people simply do not know it exists or know much about it (it wasn't as heavily employed by the G.'.D.'. as the Watchtower material was). Some of this material sneaks its way into the next section, however, so it is an area that will require further study once the student has made some progress with the Watchtowers and Aethyrs. In the O.'.S.'.D.'.L.'. we usually reserved this work for higher grades, and this is likewise for the Golden Dawn.
2. Enochian "Proper" - Language, Watchtowers, & Aethyrs
This is the bit we're all familiar with, and it includes the actual language itself, the Great Table (and its subdivisions into Watchtowers, etc.), all of the Calls/Keys, the Aethyr names, and anything else that we traditionally think of when it comes to Enochian (barring the Sigillum Dei Aemeth, which is from category 1, as noted above). Most G.'.D.'. Enochian focuses on this material.
3. Liber Loagaeth & Other Obscurities
There was other material delivered, most of which is incredibly obscure, such as the Liber Loagaeth. Most people just don't know what this is, what it is intended to do, or how to make any practical (or even theoretical) use of it. There were myriads of tables delivered and some people have some theories on their use, but this is the section that is most open to interpretation and experimentation (and also, because of this, the most potentially risky, if one manages to even get anything out of it at all). It can be linked to the other two sections (afterall, they make up one unified system in the end), but how it links up remains theoretical and sketchy at best.