Connection Types are the many ways to connect LEGO parts together using the various connection systems. Some of these systems, such as Stud and Tube are official LEGO ones, while others, such as Tile Edge and Stud Pair are fan devised, or at least, are not often seen in LEGO sets.
There is a category called category:Connection types which collects known ways to connect elements. This category eventually should have articles on every known connection type. A few stubs have been started to show the general idea, but the stubs themselves are incomplete, they give verbals but should use renderings of the parts for clarity. You are invited to expand this category by adding new types or by improving the types already there.