In America, we have 5 (should be only 4, but that's another discussion) branches of our military. Army, Navy, Air Force, Marines, Coast Guard. (Ok, back to the other discussion: The Coast Guard only counts since we are in a "time of war". Normally the Coast Guard is Dept of Transportation).
The Marines started out as Salty pointed out: the landing units of the Navy. However, they evolved into their own branch (even though their seal still says "A Dept of the Navy". They get mad when you point this out!)
Soldier is a term used to describe those in the Army (such as myself).
Sailor for Navy, Airman for Airforce. (Not sure what Coast Guard goes by, lol.)
One of the reasons there is so much contention about being called a Soldier vs a Marine is that our jobs very often overlap one another.
Thanks for the explanation. I am not familiar with the USA's defence forces since I am an Aussie.
I see that you're in Afghanistan. I hope all the US soldiers keeping safe and well especially since I bet it's getting cold. My husband may have been sent to Afghanistan last year but his medical grading wasn't quite up to scratch (recovering from knee surgery a few months earlier).