By law, the Secret Service is authorized to protect:

The president, the vice president, (or other individuals next in order of succession to the Office of the President), the president-elect and vice president-elect
The immediate families of the above individuals
Former presidents, their spouses, except when the spouse re-marries
Children of former presidents until age 16
Visiting heads of foreign states or governments and their spouses traveling with them, other distinguished foreign visitors to the United States, and official representatives of the United States performing special missions abroad
Major presidential and vice presidential candidates, and their spouses within 120 days of a general presidential election
Other individuals as designated per Executive Order of the President and
National Special Security Events, when designated as such by the Secretary of the Department of Homeland Security