Immortals
Immortals live a normal life until their first death, which is always a violent one — they don't get sick. Their origins are unknown; they are always foundling children and they cannot have children.

They don't know they were born Immortals until their first death; veteran Immortals are able to recognize a pre-Immortal.

After the first death, they stop aging — if an Immortal is 15 years old at the time of his/her first death, he/she will be a teen forever.

The Buzz
Immortals sense another person of their kind approaching.

How an Immortal Dies
Immortals die when they are beheaded — by Immortals, mortals or accidentally (in a plane crash, for instance). When Immortals are wounded, the wound heals immediately, unless it be a severe one, which takes a little more time to heal; and they feel pain. However, if the wound is in the neck or head, it heals like in mortals and the Immortal gets a scar. After they "die", it takes several minutes to them to return back to life.

The Quickening
When an Immortal beheads another, he/she gets his/her energy in a process called Quickening. The older they are, the more energy they release when they are beheaded (it is implied that the more an Immortal lives, the wiser and more powerful he/she gets). When good Immortals kill the evil ones, they get their energy, but not their wickedness. However, there is the Dark Quickening, when all wickedness is transferred, causing a lot of trouble for those who receive it.

The Game/There Can Be Only One
Immortals live according to their own rules: in the end, there can be only one. There are good and evil Immortals. They always carry a sword to be ready to fight and they cannot fight in Holy Ground (churches, cemeteries, monasteries etc). Nobody can interfere with a fight after it begins. It is considered cheating to shoot or stab to death the opponent before beheading him/her.

The Gathering/The Prize
There will be a time when all Immortals left will feel drawn to an unknown land, for The Gathering. Then, the final combat will take place; the Immortal who survives will get The Prize: the power of all Immortals who ever lived.

The Watchers
A secret society of men and women who observe and record Immortals' lives, but never interfere. It was created by Ammaletu the Akkadian after he saw Gilgamesh coming back to life, 4,000 years ago. Immortals know nothing about the Watchers — except for just a few of them. They can be identified by a tattoo in the wrist:


*Credit to this site for the information.