If you haven't guessed by now,
Platoon is based on the popular Oliver Stone movie of the same name. It doesn't exactly parallel the movie's storyline, but it does take place in Vietnam and everything revolves around squad based combat. Unfortunately, though, Firefights remain on a smaller scale than those of the movie, and they are noticeably less intense.
Usually given a hero and a squad or two of riflemen, you must follow orders (which are usually to go out and kill some Vietcong squads) and execute them with the least amount of casualties to your men. Your hero leads the rest of the squads, and if he is killed then the mission is over.
Terrain plays an important role in the completion of your missions. Flat, barren ground will yield no cover for your men, and if they are caught out in the open during a firefight, the enemy will make short work of them. Scouting ahead to look for ambushes is crucial, but there will be times when you can set up your own nasty traps.
Despite the Vietnam appeal, there is very little replay value to Platoon. The Campaign mode is short and monotonous, and the Multiplayer mode sports very few maps and is good for only Deathmatches. The weaponry in the game is varied well, as you can control guys with bazookas and even a tank or APC, but the gameplay doesn't lend itself to make use of these features.