This is the Relation presentation from DrupalCity, DrupalCamp Berlin, summarizing the background of, anatomy of, and pretty much the usage of Relation module.
-What problem space Relation wants to solve.
-How reference fields works today, and their limitations
-How these limitations have been treated previously
-Some nice basic properties of Relation, solving these problems
-The anatomy of a relation entity
-Some limitations and drawbacks of Relation
-Some future plans for Relation
-Creating a (symmetric) relation type