system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What is the difference between the army and the marines?
gpt
Traditionally, Marines served onboard ships and fought with small arms/hand to hand with the crew of the other ships.  In a sense, they were the soldiers of the ship.

Nowadays, Marines are typically the first group into action; they are a primary invasion force.  Let's say the USA wanted to invade a country.  The Marines are the first to attack, typically by sea.  Marine divisions are organized so they can deploy within a certain amount time to any part of the world, invade a location, and hold it for a brief time before getting relieved.  Once the area is under the control by the Marines, the Army takes over because the Army is better structured for long engagements.