The Original Family

The Originals are pure-blood vampires, and also are the first vampires in existence and as such, all vampires are blood-descended from them. They are also significantly much more powerful than any other vampire created since then. On the show, it was revealed that there are nine originals, they are blood relatives, and they come from Eastern Europe where they were wealthy land owners.

Parents

Mikael (Original Father)
Esther (Original Mother)

Children

Elijah
NiKlaus
Rebekah
Henrik

The Original Family of vampires was created because Mikael wanted to be stronger then the werewolves to defend themselves against because Henrik was killed by one when he and Klaus went out to see what they looked like one night when they should have stayed inside.

Esther is a witch and therefore cannot be a vampire as well so she chose to stay a witch.  The original family must be invited into your home otherwise they cannot enter.  The can be burned by sunlight but Esther cast a spell to help them be able to be in the sun much like Stefan Caroline and Damon.

What can kill or hurt an Original?

The Sun and the White Oak Tree which was part of the spell that created the originals can only kill them.  Vervain hurts vampires because it was growing at the base of the White Oak Tree that was used in creating the spell to turn the Originals.