Now, you may not know what Delta Green is. That's nothing to be ashamed of really. If you don't regularly play table-top role-playing games I could see how it would pass before your eyes, but it's high time you learned. Don't worry, I've got all you need right here.
Delta Green is similar to other table-top role-playing games (think Dungeons and Dragons) in that it's usually played with a group of friends, pencils and paper (including that most essential piece of paper: the character sheet), a few rule books, and a handful of multi-sided dice. However unlike most of the more rules-centric role-playing games, Delta Green focuses on story-telling rather than esoteric tables found in endless appendices. Another unique trait is the frailty of the characters you play. You play an average human (albeit a well-trained one) fighting an endless war against extra-dimensional beings so powerful they are often referred to a "Gods."
Why is this fun? Well, when things go wrong (and they inevitably go wrong) it's often in highly disastrous (and disturbingly amusing) ways. When you're not expiring in surprisingly macabre manners, you're going insane, developing long-lasting and debilitating psychotic behavior. It's horror gaming at it's best and it results in unique and unlikely scenarios.
The first core-rulebook publication of Delta Green happened in 1997. However, as a concept, it had already been fully fleshed-out in the gaming periodicals The Unspeakable Oath. The idea originated with John Tynes. One of the best interviews on the origins of Delta Green, and his career can be heard here. I've often described the game to people as like The X-Files but with "Lovecraftian Horrors" in the place of aliens.
Since it uses the BRP (Chaosium's "Basic Role-Playing") system and is heavily influenced by the Call of Cthulhu Role-playing game it can be thought of as Modern Call of Cthulhu. However, there are some major distinctions. For instance, some of the scenarios, due to the fact that they came-out in the '90's or because they were specifically written that way, take place during that same period. Additionally, Delta Green tales inevitably revolve around multiple vast and dangerous government conspiracies (ideas which populated and occupied a pre-9/11 United States).