Master
A master is generally a magi who has summoned a servant. They generally provide a servant with support through spells though some take front and center in fights though it is rare and generally a poor decision... A master also provides their servant with prana to stay in this world, meaning if a servant uses powerful skills, it is the master that will come out drained. The final and only physical mark of a master is their three Command Spells which generally appear on their forearm or backhand. With a command spell a master can give one order that the servant MUST follow. Broad or vague orders generally have little impact on a servant though and can be ignored such as "Do everything I say". The most common uses for command spells are things such as "Put your all into this next attack." or "Save me." Since the command spell forces absolute obedience, some things that were not possible before can become possible such as a servant teleporting large distances to rescue a master even though they have no powers over space or time.

Servant
A servant is a being that the world recognizes as a heroic spirit and are stronger the more the people of the earth know of them. They need not actually exist, but that also doesn't mean they must be fictional. A heroic spirit can also be an "Anti-Hero", an anti-hero is one that has been condemned by others for others prosperity. In other words, any sacrificial lamb can be called an anti-hero. If a being once existed it is not guaranteed that they will be summoned as they were, since a heroic spirit is what the world recognizes as such, they would be summoned based on how people want to see them. They would be far stronger than a normal human even if their abilities in their previous life was just that of a normal human. It should also be noted that a heroic spirit has no set time boundary, basically even if they are from the future, since they exist outside of time they can be summoned in the past. A hero's trademark is their noble phantasm...

Noble Phantasms
A noble phantasm is the trademark of a noble spirit. They are generally weapons such as King Arthurs Ex Calibur or Cu Chulains Gae Bolg. It is not uncommon for a heroic spirit to have a few noble phantasms either, though they were lost, King Arthur at one point also possessed the sheath to the Ex Calibur and the sword Caliburn. It is the Noble Phantasm of a heroic spirit that can generally turn the tables in a battle, even if a no name heroic spirit is facing a well known heroic spirit that should be much stronger than them, a strong enough noble phantasm can easily win the match, a good example of this is Gilgamesh's Gate of Babylon which contained all of the treasures of the world at one point meaning he has all Noble Phantasms before they became famous.It should also be noted that a noble phantasm doesn't necessarily have to be a weapon such as Hercule's God Hand which grants him eleven extra lives and impenetrable skin which was a gift granted to him after his twelve labors to repent for his sins in his life.