What is Deployment?

At some point in almost every Soldier’s Army career, his or her unit will be sent to an area of operations, usually on foreign soil. This transitioning of personnel and supplies is called deployment.

Depending on factors like the function of the Soldier’s unit, or the country’s current political situation, these missions may entail combat. However, U.S. Soldiers could also be deployed to various regions for disaster relief, humanitarian efforts, or for a variety of other reasons. Depending on the reason and the needs of the Army, deployment can last up to two years.

 http://www.goarmy.com/parents/what-to-expect/deployment.html