Fatigues are the casual clothes that military personnel wear. Fatigues sometimes have a camouflage pattern.
When you see Army soldiers wearing camouflage trousers and jackets, you can say they've got their fatigues on. Another name for fatigues is "battledress," as opposed to the more formal dress uniforms worn by members of all military branches. Fatigues are what soldiers wear when they're working or engaging in battle. In the 1770's, fatigues meant "extra duties of a soldier," from fatigue, or tiredness. It came to also mean "military clothing" in the mid-1880's.