Type I is an oblique fracture through the upper part of the odontoid process. This fracture is normailly stable and can be treated with a hard cervical collar.
Type II is a fracture occurring at the base of the odontoid as it attaches to the body of C2. These fractures can be treated surgically, or conservatively with hard collar or a halo brace
Type III fractures occurs when the fracture line extends through the body of the axis. These fractures are normally treated surgically with or without a halo brace.
References
Boyarsky I, Godorov G. C2 Fractures. eMedicine. http://emedicine.medscape.com/article/1267150-overview
