The Umbrella Corporation is the multinational pharmaceutical company responsible for the mutagenic T-and G-Viri, and the outbreaks of said viri across the globe. The company started up in Raccoon City in the '60s to mask questionable activities done under the watchful eyes of founders Ozwell Spencer and Edward Ashford.
Most campaigns will have Umbrella as the main enemy, since it is involved in every Resident Evil game to date (or at least mentioned) and as such, is easy to weave into the scenarios.
Umbrella's corporate logo.