Well, Marines are a division of the Navy... so they have been trained in the customs of Naval Infantry. They are trained in beach landings, how to board and disembark a ship, hand to hand combat and other specialties that the Army does not get standard in training. Their training is much more strenuous (being a product of the Army, I can still say this) both physically and mentally. They are generally chosen for the most strenuous duty because of this (which includes embassy guards). Their slogan is, "Semper Fi" short for Semper Fidelis which in Latin means, "Always Faithful". The slogan in my Army Infantry Unit was "Semper Primus" which in Latin means, "Always First"