The Nexus is a place where all realities meet. It is the in-between realm of the Realms of Disarray and Order. It is a neutral realm that slowly changes to suit those who arrive. It, unlike the Realms of Disarray and Order, doesn't change size in accordance to its population. It is the smallest realm of the three.
Scientists suggest it is a plane parallel to the 'real' world that may be reached by achieving a high rate of vibration with electricity, thus causing a small tear in 'reality' leading to the Nexus.
Spiritualists believe that the Nexus is the place where souls go when the body has died. They believe that it can be reached either through death or deep meditation.
In reality, both explanations are true in their own right. The Nexus is a parallel plane with a high concentration of energy that draws souls to it. It may be reached by all the ways mentioned above, and other, riskier, ways may exist. What neither scientist nor spiritualist realize is that, once you enter the Nexus, you may never return to the 'real' world. Your body there is left soulless and dead.
Souls caught in the Nexus often go to either the Realm of Disarray or the Realm of Order; few stay in the Nexus itself due to its small size and ever-changing state. Souls are divided into three groups: the Lucid, the Lurkers and the Sleepwalkers.
The Lucid are those who have recently arrived, for the most part. They remember all or most of their life in the 'real' world and still have a sense of self. Most are still searching for a way out.
The Lurkers are those who have been in the Nexus's grasp long enough for most of their memories of the 'real' world to have eroded away. They are fighting to maintain their sense of self and the remainder of their memories. Most have resigned themselves to forever being stuck there, but they still search almost mechanically for an escape.
The Sleepwalkers are those who've forgotten they'd ever been somewhere separate from the Nexus and the Realms of Disarray and Order. They go about 'life' without realizing they had a life before. They usually are shop owners. Often times Lucid and Lurkers become Sleepwalkers by trying to settle down and losing their drive to hold onto the past and escape. Those who didn't seek the Nexus, but rather died normally and were drawn in, are almost always Sleepwalkers, sometimes Lurkers, and next-to-never Lucid. They are near-mindless.
Although it may seem that distinctions between these categories are made depending on the amount of memories a soul has remaining, the main difference between them is how integrated into the Nexus's flow they are. The Lucid resist it avidly, the Lurkers resent it and only accept their new reality begrudgingly, and the Sleepwalkers are immersed in it and accept it as the only reality with (usually) no resistance.