Extent of herniation. It is the downward displacement of the cerebellum, specifically a region called the "tonsils", through the foramen magnum. There are actually 4 types. Type one is only tonsillar herniation, that is mild; type ii shows more herniation, to include a middle portion of the cerebellum. This type is often associated with an myelomeningocele (herniation of lining of the brain).
Huge difference. Chiari I is just low lying cerebellum tonsils which is usually incidental and not relevant. Chiari ii is a complex found in patients with spina bifida (often called arnold-chiari) including kinking of medulla, obstruction of ivth ventricle, peaking of quadrigeminal plate, fusion of thalamus, hydrocephalus, and associated with lack of closure of the spinal canal.