Masochism is getting sexually aroused by fantasies about being humiliated, beaten, tied up or otherwise tortured. Sadism is getting sexually aroused by incurring humiliation, beating, tying up or otherwise torturing. Sado-Masochism is the combination of Sadism and Masochism in a complementary manner.
Masochism and Sadism are often caused by having been beaten or otherwise submitted to sadism as a child. This risk is a strong argument against even beating children.