Knowledge and training in the Force passes down through master/apprentice relationships. These include the formal master/Padawan relationships common in the Prequel era and the informal training relationships common in Luke's New Jedi Order. Observing the map of relationships between masters and apprentices helps reveal how Star Wars characters are connected -- and how they became the Jedi or Sith that they are.
This first chart shows the relationships between the nine main Sith and Jedi characters in the Star Wars films. The red lines indicate Sith or Dark Jedi apprenticeships, while the green lines indicate Jedi apprenticeships.
Only three of the relationships shown occur in the Expanded Universe. First, a deleted scene in Revenge of the Sith (and the Revenge of the Sith novelization) explains how Qui-Gon Jinn appeared to Yoda as a Force ghost and became his new master. This post-mortem training from Qui-Gon is how Yoda and Obi-Wan learned to become Force ghosts themselves.
Second, Princess Leia was not trained as a Jedi in the Star Wars films, since she only found out about her heritage in Return of the Jedi. Luke provided some training for her while she continued in her role as a politician, but it was a while before she began training more intensely and officially joined the Jedi Order.
Finally, Luke was briefly Emperor Palpatine's apprentice in the comic Dark Empire, which featured Palpatine returning to life in a cloned body.
Charting the rest of the master/apprentice relationships in the Expanded Universe, however, quickly becomes problematic.