It changes how long you go without feeding as a vampire. At normal you get a 5+ increase to Strength, Willpower, Speed, Acrobatics, Athletics, Destruction, Hand-to-Hand, Illusion, Mysticism and Speed and also a bonus to Resist Disease, Paralysis and Normal Weapons. Everyday without feeding these increases go up by 5+. However, as the days go by, you weakness to fire goes up and so does sun damage. The powers you get are as follows:
Day 1 - Hunters Sight: 30 secs of Night-E and large Detect Life
Day 2 - Vampire's Seduction: High level Charm
Day 3 - Reign of Terror: Silence and Demorilize around you
Day 4 - Embrace of Shadows: Three minutes of Invisibility and 90 secs of Night-E