The Nexus is a nonlinear temporal phenomenon which can be entered via a temporal flux energy ribbon which crosses the Galaxy every 39 years. It is possible that the ribbon was created by a young Q billions of years ago. (TNG novel: Q-Space)
First known contact with the Nexus was made in 2293 by a group of El-Aurian refugees, including Guinan and Tolian Soran. The refugees were rescued by the USS Enterprise-B, but at the apparent cost of the life of James T. Kirk. (TNG movie: Star Trek Generations)
In 2332, Guinan was visiting Oblivion when the Nexus energy ribbon again crossed through the Galaxy. She felt it even 3,000 kilometers from the orbital city, reopening old wounds for her. (STA novel: Oblivion)
In 2371, Soran, having become obsessed with returning to the Nexus, committed several criminal acts in an attempt to bring the Nexus to Veridian III, having determined there was no guaranteed safe way to approach it by ship. Soran was defeated by Captain Jean-Luc Picard and Kirk, although doing so cost Kirk his life. (TNG movie: Star Trek Generations)
It appears that the Nexus is responsible for the strange 'feelings' that have guided Guinan for so long, as the aspect of herself that remains in the Nexus is thus aware of all times simultaneously, and can reach out to her other selves and prompt them along the correct path.
The Nexus in Alternate TimelinesEdit
In one alternate timeline, Montgomery Scott traveled from 2370 to 2293 to beam Kirk from the Enterprise-B just before he was pulled into the Nexus. This altered history so that the Borg invasion of 2063 Earth was successful; without Kirk's aid, Picard either failed to stop Soran or died in the attempt, and Earth was thus conquered before humanity could develop Starfleet. Fortunately, the Enterprise-D had followed Scott on his journey to the past and were thus protected from the changes in history that had occurred while they were 'in transit'. On the advice of Guinan and the Guardian of Forever, the Enterprise was thus able to send Kirk into the Nexus moments before they were destroyed by a Borg cube, undoing Scott's original rescue of him and restoring history to its proper course. (TNG novel: Engines of Destiny)