The earliest American Marines were based on their British counterparts, although the modern USMC is a quite different thing. For one, the Royal Marines are elite light infantry, whereas the USMC are primarily a shock force (with integral close air support, artillery, heavy armor and motor transport abilities but oddly missing Combat Medic personnelnote ).
The present role of the USMC is to serve as a multipurpose rapid response force, so at any given time they are more likely to see action even in supposedly "peaceful" times. However, the original purpose of "marines" was to provide an onboard force of infantry for wooden sailing vessels, both to keep order among the crew, and to form the core of a Boarding Party when that was still a viable tactic in naval warfare. Before this, a ship's crew would be assumed to handle both their shipboard duties and fighting, but later they specialized into crew members who handled the ship and marines who handled the fighting. This distinction is, at the very least, Older Than Feudalism: the Roman Navy had several dedicated marine legions that could be deployed on ships or as auxiliary units on land. The current U.S. Marines still have their naval traditions, as well as patrolling with the world's largest amphibious fleet (by far) provided by the Navy to this day, but generally just consider themselves the first troops to go into the battle, wherever it is.
The U.S. Marine's official motto, "Semper Fidelis" means "Always Faithful" in Latin. It is commonly shortened to "Semper Fi", hence the trope name.
If you happen to run across a now-civilian who served in the USMC, be careful that you don't address them as an ex-Marine; that ignoble distinction is (usually) reserved for personnel who receive a dishonorable discharge from the Corps. In one Marine's words , "The only ex-Marines are Lee Harvey Oswald and Charles Whitman." The title of Marine is permanent once it is earned, as long as it is not revoked for dishonorable behavior. Former or Veteran Marine is preferred, though if you know it you can also use their last achieved rank before leaving the service. Retired Marine works too, depending on the context - it's kind of odd to refer to a 25-year-old currently working in the same place as you as "retired." The Marines' attitude is summed up by this statement: "Once a Marine, always a Marine".
Don't call 'em "soldier". Most of 'em don't like that, either. Don't try debating them at it, either; it's a whole thing. For that matter, actual soldiers (that is, those service-members who are in the US Army) will take offense to you calling Marines "soldiers" as well, because "they ain't soldiers, they're just Marines."
