The Schedule Caste people are the one who were previously untouchables. According the Hindu mythology, this is the fifth category in the Varna System. They are called Ati Shudras (Untouchables) and were condemned for all dirty and polluting jobs. They named themselves as Dalits or Harijans (son of God).
On the other hand, Schedule Tribes are community of people who lived in tribal areas (mainly forest). They make up to 7-8% of Indian population. They have traditionally been marginalised and not in the mainstream of the society. They are also known as Adivasis.