Aeris
As explained in "Final Fantasy VII," Aeris held onto a mysterious stone since she was a baby. It belonged to her mother Ifalna, the last of the Ancients. This stone was the White Materia, otherwise called Holy. This was the ultimate defensive magic of the Ancients. Aeris kept the White Materia throughout the game until Sephiroth killed her in the Forgotten Capital.
Holy
When Aeris died, she joined the Lifestream. However, the prayer she had given to the Ancients awoke the White Materia. Instead of protecting the planet from the meteor that Sephiroth had brought down with the Black Materia, it accelerated it, hoping to wipe out the humans that it saw as tainting the planet. Only Aeris channeling the Lifestream saved the planet.
Sephiroth
Sephiroth wanted to be a god and rule at his surrogate mother Jenova's side when he found out that he was nothing but a genetic experiment of Shinra. His ultimate goal was to find the Black Materia. Whereas Holy was the Ancients' ultimate defensive magic, the Black Materia, or Meteor, was his ultimate offensive magic. It captured a small planet and pulled it toward the earth. Sephiroth intended to use the power of the Lifestream, meant to heal the planet, to become godlike.
Black Materia
Sephiroth found the Black Materia hidden in the Temple of the Ancients. Although character Cloud Strife and his company shrank the Black Materia/Temple of the Ancients and intended to hide it away, Cloud's weakness caused him to give in to Sephiroth's goading to give him the Black Materia, which he then used to summon Meteor to the planet.