Old School Renaissance is a movement in RPGing where old school games come back to life with their systems and their way of playing them. Mostly centered around (A)D&D - referred as OD&D (Original Dungeon & Dragons) - they led to the creation of retroclones by small independent designers. These retroclones are game system that are compatible with these older editions, working around issues of copyright and trademark.
OSR is a lot about going in the opposite direction of the current of narrativist games - a different audience, albeit not completely distinct. I personaly like both approach, differently.
Here are some link about OD&D and its retroclones:
An illustrated guide of D&D editions: This simple page should help you identify the editions of D&D and associated retroclones to them.
D&D Retroclones: The various D&D retroclones and variants using retrocloned rule systems.
Innovation and the Old School Renaissance: since some people tend to equate “OSR” with nothing new, this articles tries to link recent development for OSR system: new settings, new classes, new rules, etc. This gives a pretty extensive list.
We’ll explore the subject a bit more in depth later.