Infinity Ward isn't necessarily the most adventerous game studio out there; after breaking off from the development team that made Medal of Honor: Allied Assault, it went on to make two more World War II first-person shooter games, Call of Duty and Call of Duty 2. Both were exceptional, but they also helped form a glut of WWII shooters that practically drowned consoles and PCs in the early part of this decade.
Now, though, Infinity Ward has updated its game engine for next-generation consoles and has also updated the setting. Call of Duty 4: Modern Combat takes place in a contemporary setting, with real-world guns and a taut storyline that draws upon the issues of the day. It's not going to revolutionize the shooter genre--in the end, you still have to point and shoot--but it's a welcome departure from the world of potato mashers and MP44s. (Well, mostly…)
GameSpot's Primer to Call of Duty 4 isn't intended to be a full game guide with a walk-through and all the works, because the game is pretty simple to play, so long as you can point a gun and pull a trigger. Instead, here's what you can expect from this primer:

Intel Locations

There are 30 pieces of Intel in the single-player game. Finding them will let you unlock new cheats for the single-player game after completing it once. They appear in the game world as laptops; to pick them up, you simply need to find the laptop and pick it up. If you miss a piece of intel, don't worry: you can try to find it again by re-playing the level it's on.
Here's a list of all 30 intel locations. A list of cheats is available after the locations.

Prologue

Crew Expendable

Intel One: This intel is located in the room the drunk soldier comes out of when you first enter the ship, near the two sleeping soldiers.

Intel Two: After clearing the deck, you'll head into the hold of the ship. You'll eventually reach the cargo holds. After Captain Price throws a flashbang through a door, follow him through and head to the bottom of the steps. The laptop is on the floor near the hole in the metal grate.

Act I

Blackout

Intel Three: At the beginning of the level, you'll come to a few Russians standing outside a shack overlooking the swamp. Head into the shack to find the laptop.
Intel Four: On the second floor of the building where you need to use night vision goggles, check the bathroom before you rescue Nikolai. The laptop is near the toilet.

Charlie Don't Surf

Intel Five: After reaching the target building at the beginning of the level, head downstairs into the room filled with weapons. A small room through a doorway from this room will have the laptop near a television set.
Intel Six: After exiting the first target building, stick to the right as best you can. You'll bypass the firefight amongst the corrugated steel. If you stick to the right, you'll eventually come to a building with a light blue interior and stairs leading up. The second floor holds the intel. If you get to the point where you fight someone in a technical (pickup truck with a machinegun in the back of it), you've gone too far.
Intel Seven: After defeating the soldier in the technical (pickup truck with a machinegun in it), you'll cross the main road. It's easy to miss, but one of the buildings on the right will have steps leading up to the second floor. Look for a poster of a red fist on the wall with a star above it; the stairs will be immediately next to it. Head up them to find the third piece of intel in the level.

The Bog

Intel Eight: In the first building that you enter, you'll be tasked with using the enemy machinegun emplacements against them. After doing so, return to the hallway of the second floor, where your soldiers will breach a door. Head through the door to reach the first piece of intel.

Intel Nine: After using the Javelin to destroy the enemy vehicles, you'll head into the alleyway between you and the Abrams. Check behind a pallette of cardboard boxes in one of the alleyway's corners to reach the next piece of intel.

Hunted

Intel Ten: Near the beginning of the level, you'll be hunted by a helicopter. After entering the basement of the building nearby, fight your way to the first floor and out the front door (where you'll likely encounter a flashbang grenade attack). After the flashbang, head outside and into the building on your right to find the laptop.
Intel 11: Eventually you'll head through two greenhouses. When Price tells you "The helicopter's making another pass!", look around for the tin roof between the second greenhouse and the barn where the Stingers are located. The laptop is underneath that roof.

War Pig

Intel 12: Near the beginning of the level, you'll escort the tank out of the bog. After it crushes the abandoned car, stick to the right side of the street. One of the buildings there will have a second story. Take the stairs up and access the laptop to find the intel.
Intel 13: When you find the first piece of intel, jump out of the building and cross the street. To the southeast, you'll find another two-story building. In one of the rooms here, you should find a cot with a laptop on it.
Intel 14: Near the end of the level, you'll reach an alley full of cars, overlooked by a pair of machineguns. Fight your way to the MGs. The room that they're in will have the laptop.

Shock And Awe

Intel 15: The halfway mark will be reached in the same room where you encounter the stranted recon team on the second floor of a building. Look to their right and you'll see the laptop behind some barrels.
Intel 16: After you meet the recon squad, you'll jump down to a firefight with a lot of troops. Some of them will be firing at you from the windows of the second story of a building. Fight your way into the building, climb the stairs to the second floor, and look into the bathroom there for the intel.