Wikipedia: Marine

A branch of a country's navy which fights on land. Often designated a corps, not a service. Marines are naval infantry, specialising in fighting at sea on ships, forward deployment, naval security and amphibious assault.

Originated from the fact that it was an act of war for a country to send its army into another country but not to send its navy.

Note that some countries do not differentiate branches of their armed forces, as [Israeli Defense Force]?, [Canadian Armed Forces]?.