Tea classification has changed over the years, but a widely accepted set of six is the following. Some classifications don't include Yellow Tea and therefore speak of only five classes of tea.
Green – no oxidation (oxidation prevented by steaming or dry heat)
Yellow – no oxidation & ‘smothered’ for smoothness
White – less than 8% oxidation
Oolong – partially oxidated, 20%-80%
Black – fully oxidated
Pu-erh – always fermented, not always oxidised
(You know when you cut an apple or a head of lettuce and it turns brown? That's oxidation. That's why "black" tea leaves are deep coppery brown/black.)