okay. ill break it down simple. and im being VERY general here.
army is like, soldier stuff, you fight with guns, in wars and all that stuff. That is millitary on land.
Navy is boat stuff. its like millitary in the sea.
and marines....a person who serves on both ship and land.
Army: the main bulk of the attack force in case of war
The Navy: Mainly Sea warfare, ships and such
Marines: Special military unit to lead force projection (aka Intimidation, annihilation etc.), a part of the US navy they use amphibious vehicles, and special techniques in warfare
Navy does warfare by air and sea.