The OSR ("Old School Renaissance") is a label usually applied to an informal, disorganized group of table-top RPG players that are interested in the hobby as it was played in the 70s and 80s. In the words of Rob Conley:
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.The best short overview is probably Matthew Finch's (free) Quick Primer for Old School Gaming.