Basic Role-Playing (BRP) is a role-playing game system which originated in the fantasy-oriented RuneQuest role-playing game rules. A percentile skill-based system, BRP was used as the basis for most of the games published by Chaosium, including Stormbringer, Call of Cthulhu, Superworld, Ringworld, Elfquest, Hawkmoon, Elric!, and Nephilim. Pendragon (acquired in 1998 by Green Knight Publishing, and 2005 by White Wolf Entertainment AB), while related, has sufficiently different mechanics that it can be seen as a separate system. The BRP standalone booklet was first released in 1980. Two years later it became part of the Worlds of Wonder boxed set. The first edition boxed set of Call of Cthulhu included the booklet as its character creation rules. Greg Stafford and Lynn Willis are credited as the authors.
BRP is similar to other generic systems such as GURPS, HERO System or Savage Worlds in that it uses a simple resolution method which can be broadly applied. In this case an attempt to roll under a certain number with percentile dice. Each incarnation of the BRP rules has changed or added to the core ideas and mechanics, so that games are not identical. For example, in Call of Cthulhu, skills may never be over 100%, while in Stormbringer skills in excess of 100% are encouraged for those who follow Law.
In 2004, Chaosium published the Basic Roleplaying Monographs (the hyphen was dropped in the later products). Books with a quick and inexpensive printed format of tape binding and printed cardstock covers, the four monographs (Players Book, Magic Book, Creatures Book, and Gamemaster Book) were printed in order to assert Chaosium's copyrights in the run-up to the publishing and distribution of Deluxe Basic Roleplaying, a game system that is essentially RuneQuest 3rd Edition but with additions to allow play in other genres.
Chaosium released a new version of BRP on June 24th, 2008 as single comprehensive book. Currently they are selling both a printed and pdf version of the game.
Source: Wikipedia, "Basic Role-Playing", available under the CC-BY-SA License.