Hi Ximena,
I strongly suggest reading this PMI article about risks, it does a good job explaining various types of risks:
LINK
In essence:
Variability risk: There is a set number of possible outcomes but we don’t know which one will actually occur.
(ex. The number of errors found during testing may be higher or lower than expected)
Ambiguity risk: Come from uncertainties arising from lack of knowledge or understanding.
(ex. New disruptive technologies or market conditions)