Bro is an open source framework for network visibility developed in 1995 and used widely in the defense, intelligence and national research universities. Although it's a popular solution in those segments, it is still less known in the enterprise market.
Until recently, the types of threats faced by enterprises were significantly less severe than those faced by military, intelligence and other government agencies. On top of that, since Bro was not developed for enterprises but rather as a research tool, it did not have the "enterprise-grade" features and functionality required.
Download this whitepaper that discusses the basic functionality of Bro, what makes it different and how it can be a powerful addition to an enterprise's network security defenses.