Generally, they were Civil Rights Activists.
More specifically, they were people from the 1960's who wanted
segregation laws changed. Many of them were members of the Congress
of Racial Equality. Freedom Riders were both African-American and
white.
Some notable Freedom Riders are: Stokely Carmichael, Roby Doris
Smith-Robinson, James Peck, James L. Farmer, Jr., James Lewis,
William Mahoney, and US Representative Bob Filner (D-CA).