The USMC was originally created to A) aid the Navy in shipboard protection back in the colonial days and B) act as an extremely well trained ground unit, trained to an extent where they are as effective in small numbers as a much larger number of regular Army personel. The Marines were to be specialists at landing/coastal operations.
The USMC's main role in war was to protect it's ship from intruders during sea battles. As you know, in those days ships would come together and exchange cannon and musket fire. To counter this, Marines would actually climb all the way to the top of their ship's rigging and saturate the enemy's decks with gunfire/grenades. This is why the USMC is known for their rifle skills, because they all had to in fact be sharpshooters to hit a target on a moving ship from that high up. To make sure there was no "friendly fire" the USMC adopted the quatrefoil, a cross shaped braid that was on top of their caps in order to tell friend from foe. To this day USMC serive caps still have a quatrefoil sewn to the top.
The USMC's main claim to fame is of course landing operations, of which they have conducted ~300 in their existence. To this day landing/spearhead operations are the USMC's bread and butter. WHile in WWII Army personel did take part in beach landings, they merely adopted the USMC's tactics and techniques and in most cases were actually trained by the USMC since the Army then and to this day does not train or have the means to train for those types of operations.
To this day the Marines act as the "spearhead" for all US military conflicts. The Army acts as a "cleanup and occupation crew" but also assists the Marines on the frontlines at some times. They also act as security for all US government installations such as the White House and all US embassies/consulates overseas. Because of this, the MSG (Marine Security Guards) are the only other military units other than Navy Seabees who fall under the command of a civilian. They also provide security for all US Naval bases as well as maintaining the tradition of providing security for all US Naval vessels.
Since the USMC is classified as a rapid reaction force, able to deploy itself to an AOR and also sustain itself using it's own equipment, it can be classified as not only an Army in itself but also a large special forces unit. In order to complete it's objectives the USMC relies on speed and violence of attack, quickly overrunning their enemies.
I think thats it lol
Some fun facts about the USMC:
*In Washington DC, the Commandant of the Marine Corps's license plate reads "1775", the year the USMC was created
*Since the 1920's, and English bulldog has been the official mascot of the Marine Barracks in Washington, DC
*All Marine posts (and some camps) have a bell, usually from a decommisioned US Navy warship
*If a Marine officer enters a mess without removing his hat, he or she is liable to buy a round of drinks for everyone present. The exception is if the officer is on duty and under arms
*The USMC celebrates it's birthday every November 10, and is the top social event of the year. The cake, if present, is cut with a mameluke sword and slices are presented by the senior Marine to the youngest and oldest Marine
*When Marines enter a boat or vehicle, the junior person goes first and takes the less desirable places in the middle or front. When debarking, the senior leaves first, while juniors follow in order of rank
*The "Marine Corps Hymn" is the oldest of the official songs of the US Armed Services
*The Mameluke sword, used by Marine officers since 1826, is the weapon with the longest service in the US Military. The second longest serving weapon is the ceremonial swords carried by Marine sergeants, a tradition that began in 1850
*The red-colored leg stripe on a Marine's Dress Blue uniform is known as the "Blood Stripe", in remembrance of the Marines who stormed Chapultec Castle in Mexico City during a bloddy battle in 1847
*Every New Year's Day since 1869, the Marine Band has serenaded the Commandant of the Marine Corps at his quarters. Afterward, the band members are invited inside for breakfast and hot-buttered rum
*If a Marine unsheathes a sword inside a mess or wardroom, he or she must buy a round of drinks for all present. This tradition helped prevent dueling in the early days of the Navy and Marine Corps
*According to Navy regulations (1865), Marines - other than the ship's captain - are always the last to leave a ship being abandoned or decommisioned
*The phrase "Tell it to the Marines" comes from King Charles II, who was shocked to learn about flying fish from a sea captain. He turned to a member of the Maritime Regiment (precursor to the Royal Marines) and asked if it was true. The Colonel confirmed it, whereupon Charles II decreed that from then on anytime a strange thing was presented "....we will tell it to the Marines, for they go everywhere and see everything, and if they say it is so, we will believe it".
*All Marines salute a Medal of Honor recipient, regardless of rank
*A Marine will fight and die rather than permit the National Colors (American Flag) or a Marine Corps color to be dishonored or captured. If capture seems likely, the flags are burned.