There are two main types of apes: "lesser apes" and "great apes". Lesser apes include mainly all gibbon species, whereas the Great apes include orangutans, gorillas, chimpanzees, and bonobos.
The Primates: Apes
The Leakey Foundation Five Fun Facts About Gibbons
The Leakey Foundation Five Fun Facts About Gibbons
Species Information - Orang Utan Republik Foundation
Types of Apes, Gibbons, Gorillas, Orangutans, Chimp
Monkeys vs Apes: How are they different? - PASA
Primate test - Apenheul
10 Fast Facts in Honor of Ape-ril - Orangutan Conservancy
Why Orangutans?