The US Marines are an amphibious expeditionary force whose main job is projection of power using the mobility of The US Navy. Basically, if you go to war with the USA, they’re the first guys your soldiers will see and they’ll be delivered by the US Navy. The Marines are capable of fighting on the land, sea and the air. They employ armored vehicles, fighter aircraft, helicopters and artillery to complete their missions. Marines are tough soldiers who are trained to act and fight as a unit. This unit cohesion is a large part of their strength because trained, determined and disciplined fighting men who are inseparable can defeat larger groups of less disciplined men.
Navy Seals on the other hand fight as small units. They perform specific missions within the battle which require finesse rather than brute force. This isn’t to say that they can’t fight, they are just tasked with a mission and will avoid fighting if it endangers the success of the mission. Like the Marines they can fight on the land, sea, and from the air, but completing a specific mission, rather than overwhelming their enemy, is their primary function. Like the Marines they also rely on unit cohesion to be successful, but are also trained to act independently when needed. I guess you could say that the Marines are hammers, sledge hammers, and there are lots of them. The SEALs, on the other hand, are surgical scalpels. Both can do damage but in a very different way.