A conscientious objector (CO) is a person who refuses to serve in the armed forces because to do so would violate his conscience. Such people are often motivated by a religious idea of pacifism, such as Quakers, Seventh Day Adventists or Jehovah's Witnesses. A CO can also be a serving member of the military that has moral objections to killing another human, yet still wants to remain in the military. If the member is in combat, they usually take a position such as medic. In fact, Desmond Doss, a Seventh Day Adventist who refused to carry a weapon, was awarded the Medal of Honor. He is the only CO to ever be awarded this honor.
Conscientious objector status is generally not given to people who have political reasons for opposing their nation's military policy, even if they call themselves "anti-war". They must demonstrate a moral or religious objection to the institution of war itself.
"Selective" conscientious objectors only refuse to fight in a particular war, but the U.S. does not grant them CO status. Nor does it grant CO status for mere philosophical or practical objections.