Servants in ancient Rome were very common in the upper class. Slaves were at the very bottom of the hierarchy. They usually consisted of people who were taken prisoner by the Roman military. These people would reproduce and then their children would be born into slavery as well. Sometimes people became slaves in order to get themselves out of debt.
Everyday life for these people consisted of very hard, laboring work. As first slaves were treated very poorly but it was discovered that if they were treated better they would work more productively. They actually were given many privileges.
Once a year there was a celebration called the Saturnalia. This was a day when the masters and servants traded roles for a day. This was how they showed loyalty to one another. By doing each others jobs, they learned to appreciate one another.
Concerning servants in Shakespeare's Julius Caesar, I'll describe one servant in the play for you.
Pindarus is a servant of Cassius. He previously was captured by Cassius, but Cassius spared his life and made him a servant. Pindarus basically does whatever Cassius asks him to do. He has a small role in the work, but has a part in two important events.
First, Cassius orders him to go to a higher vantage point from which he can see the fighting, and to report back to Cassius what he sees.
Second, Cassius asks Pindarus, in exchange for his freedom, to run Cassius through with Cassius's own sword, once Cassius thinks the battle is lost.
Pindarus, though a slave and servant, seems extremely fond of and loyal to Cassius, and in fact warns him to escape before the battle reaches him and puts him in danger. Cassius refuses to flee.
At the same time, once Cassius is dead, Pindarus doesn't wait around for the enemy to arrive, and he doesn't choose to die with his master. He does flee.
Finally, though as far as the reader knows this is not really a reflection on Pindarus or his attitude toward Cassius, he makes the error in perception or judgment that makes Cassius think the battle is lost. When Pindarus gains a better vantage point and sees the battle, he thinks he sees one thing but in fact sees another. When he reports events to Cassius, Cassius thinks his friend has been captured and the battle is lost.