A platoon is an organizational unit in the US Army, US Marines, US Navy, and US Coast Guard1. Each of these units is made up of two to four squads or sections for a total of 16-44 soldiers depending on the branch of service and unit type. There are three to five platoons in a company each of which perform a specific duty or fulfill a specific mission. It is most common for a platoon to directly support its company, though they can be sent out on independent missions as required.
A platoon is typically lead by two individuals: a Platoon Leader and a Platoon Sergeant. The Platoon Leader is a Commissioned Officer usually a First or Second Lieutenant (Ensign or Lieutenant Junior Grade in the Navy), while the Platoon Sergeant is a Noncommissioned Officer (NCO) usually holding the grade of E-7.
While it is significantly more complex than this, in general the Platoon Leader usually just gives orders based on the information he receives from his Company Commander, and has many more responsibilities while his soldiers are in the field or at war. The Platoon Sergeant works with and leads the soldiers on a day-to-day basis and is usually the primary leader of his soldiers while they are in garrison.
See also: United States Military Chain of Command
1 The closest organizational unit to a platoon in the US Air Force is a flight, however flights are usually larger than platoons, and are commonly more on scale with a company.