Mortals are ordinary human beings that have no magical or supernatural abilities or heritage. Most mortals are entirely unaware of the existence of magic. However, some mortals learn of the magical world when they meet someone magical or become targets for demons who seek to harm innocents.
The term mortal may be somewhat misleading, as it indicates only a lack of participation in the magical world, and not the ability to suffer death. Most of the supernatural beings depicted throughout the series have material bodies and are capable of dying, but are never referred to as mortal.
In the early days of humanity, a woman came upon the Nexus of the All, a spiritual energy that runs through everything. Though fearing it's power at first, she came to pass over it and taped into the All's power along with her mate. This created a realm beyond the physical universe: the Higher Realm, where they came to live in.
But their bond with the All caused the Earth to slowly die and they were forced to return. The woman gave birth to two children, a witch and a warlock, who were concieved while in the Higher Realm. The children that came later possessed no power to speak of but they could tap into magic as witch practioneers. 
In the times of Ancient Greece, a race of God-like beings named Titans ruled the world and mortals worshiped them in hopes of appeasing them. The Titans were eventualy defeated by the Greek Gods who equally forced the world to worship them until the Elders stripped them of their powers. 
In the centuries that followed mortals came to fear magic and persecuted it in horrific Witch Trials, causing both Good and Evil to decide to hide their supernatural nature.  However, recently, magic switched and mortals were granted supernatural powers. 
Mortals and the NexusEdit
In "Scry Hard", it was revealed that mortals are neutral beings, or are at least considered to be neutral by the Nexus. This is proven by the fact that when the Nexus had to choose between good and evil, the Charmed Ones and Zankou respectively, it instead chose the then mortal Leo as neutral ground.
Mortals and PowersEdit
Mortals are capable of becoming magical through various means. They can become demons through various forms of Demonic Conversion and can be imbued with powers through spells or power granting. When a good person dies, he can earn his right to become a whitelighter depending on how much good he has done in his life.
However, there have been exceptions: Curtis Williamson gained powers of the Charmed Ones when he injected himself with their blood, but the overload of power led to insanity and loss of mercy and compassion; in another case, Karen Young was hit by a ball of demonic powers when a group of demons called Power Brokers chose her as their test subject. This led her to paranoia, insanity and almost death before the sisters removed the powers from her.
In Season 9, the shift of magic causes all magical beings to lose their powers and mortals to develop them. All of them seemingly have found ways to handle their powers, but a society of magic causes great chaos to the world.
Over the course of the series, the Charmed Ones have entrusted several mortals with the secret that they were witches, including friends, loved ones and innocents. For a complete list, see the main paige on Secret Keepers.