A social reformer is anyone who advocates for reform of a certain area of society. "Crusader" and "meliorist" are used as general synonyms for social reformers.
There are many different types of social reformers who aim to correct different problems that society faces. Abolitionists, or emancipists, for example, were social reformers who focused on putting an end to slavery. A person who begins many disputes and enjoys controversy is known as a controversialist. Birth control campaigners or reformers are a modern day example of a social reform movement that works to allow birth control and family planning for all people. Reformers who focus on ending racial stereotypes in society are known as civil rights activists.
In the 19th century in England, chartists were social reformers who advocated for better economic and social conditions for workers. Prior to Prohibition, prohibitionists were those who wanted to outlaw the production of alcohol in the country.
Another modern day reformer is an environmentalist, who aims to protect the environment. A women's rightist advocates for equal rights for women. Freedom fighters and rebels are also normally considered social reformists. On the other end of the spectrum are pacifists, who want society to change but do not believe in using violence as a means for change.