Infantry are the men and women who actually fight on the ground.
The UK has lots of different types of troops, including 6,000 parachute troops and 3,500 Royal Marines, who usually go in first.
America has around 170,000 marines in the Gulf, who are their main fighting force. They also have a number of Special Forces.
Iraq is also thought to have around 170,000 troops, but most of them are ordinary people ordered to become soldiers by the government.
That means a lot of them may refuse to fight when war starts, and just give up.
But there are also 60,000 Republican Guards, Saddam's best soldiers.
Their training and equipment is not as advanced as that of a normal US or UK soldier.