What is the actural cause of dizziness or vertigo?

Many causes. Vertigo is a sensation of you or your environment spinning. This implies dysfunction in the balance system, from brainstem to inner ear. Obviously many causes including toxins, drugs, infection, brain injury, stroke, tumor, irritating blood vessels, congenital anomalies & many, many more. Dizziness is a vague, ill-defined term which has therefor a much broader range of possible causes.
Vertigo. The structures mediating vertigo and dizziness are in the inner ear. Inflammation, trauma and reaction to some medications may be causative.