Among other factors, Dandruff is caused by sebaceous gland activity and opportunistic fungi called Malassezia. These fungi tend to colonize areas of skin that are high in oil (sebum), like the scalp, face, and upper body. Anti-dandruff shampoos like Nizoral work by preventing these fungi from growing.
There's no cure for dandruff, but it's very treatable, and its symptoms tend to improve in warmer months.