A modification (or "mod") for Unreal Tournament. Infiltration is one of several mods intended to make first person shooters more realistic. Many FPS games (including unmodified Unreal Tournament) tend to be very unrealistic, with super-tough, super-fast characters running at super-high speeds around fantastic or futuristic levels, firing such weapons as railguns and rocket launchers at each other. Techniques such as the rocket jump, which would be utterly suicidal in reality, are common in such games. Infiltration, and other realism-oriented mods, attempt to get away from this.
First of all, the weapons used in Infiltration are all real-world firearms, from the Desert Eagle handgun to the M16 Assault Rifle, these weapons are based closely on their real-world counterparts. For instance, the M16A2, the M16 varient used in the game, has only semi-auto and 3-round burst fire, and so does the version in the game. You can add attachments such as grenade launchers and sniper scopes to your guns, as in real life. Perhaps the most realistic feature of the guns is the fact that you actually aim them they way you aim a real gun. Pressing the alternate fire button brings the gun up to the center of the screen, allowing you to aim using the gun's sights. This is necessary, because there is no crosshair in the game (unless, of course, you look through the scope of a gun equipped with one).
In addition to the guns, the game's physics are more realistic than most shooters. For instance, you can't carry one of each gun, with full ammo for all, like you can in most FPS games. The more gear you have, the less stamina you have, and such activities as running, jumping, and diving for cover all require stamina. You can't load up and expect to be able to dart around madly, jumping and shooting. Or rather, you can, but you won't be able to run far, and you can't hit the broad side of a barn. If you do dash, you can't immediately stop, but have to decelerate instead. Run out of stamina and the best you can do is a brisk walk.
Another big difference is the notion of loadout. You don't just spawn in the level with just a pistol, and have to grab the spinning guns that float in midair. Rather, you choose what weapons to start out with, on a loadout screen. For instance, a sniper might choose to carry a PSG-1 rifle, with a handgun for personal defense, while an assault-oriented player might choose an automatic rifle and plenty of hand grenades. This isn't like the weapon-buying system in Counterstrike. You don't win money to buy better weapons. It's just there to simulate the fact that soldiers go into battle fully-equipped, and there are no level designers in real life who thoughtfully place weapons on the battlefield to be picked up and used by whoever needs them.
Despite all this, Infiltration isn't the perfect real-world combat simulator, at least not yet. The most glaring problem is the gameplay itself. Right now, Infiltration is basically Team Deathmatch with only one life. The goal of each team is to eliminate all members of the other team. As a result, players often play like any other TDM game, with an emphasis on personal skill rather than teamwork. You can't rocket jump or leap over the head of an enemy in Infiltration, but there are plenty of people who just charge in, guns blazing, and with quick enough reflexes and a DSL connection, that can actually work. However, new game modes are intended, including the intriguing EAS {Enhanced Assault) mode, which should offer complex, strategic objectives more similar to real-world military objectives, than plain deathmatch.
Infiltration still has a ways to go before it fulfills all it sets out to do, but it's well on the way. The guns are great, the physics are good, but need improvement, and the gameplay needs signficant improvement, but all of these improvements and more are planned. In all, I recommend that anyone who owns Unreal Tournament, and has any interest at all in military or covert ops mods such as Counterstrike, at least consider downloading it (you can get it at http://www.planetunreal.com/infiltration/). It's a big file if you have a modem connection, but it's worth the download.