Answer:
Shudra is a Hindu term. Untouchable is a term coined by the British. While all untouchables are Shudras all Shudras are not be an untouchables. Shudras are integral part of Hindu society but untouchables are not.
THE SHUDRA:
Shudra is a Varna and also a person using that Varna. This Varna is used for survival when living conditions are chaotic.
In this Varna,
1. All activities are independent of each other
2. The status of every activity keeps varying without limits
3. Every activity of life is transient. He can't have a permanent occupation or an asset.
To lead a better quality of life a Shudra has to stabilize his life. He has to be under the guidance of a person of a higher Varna.
Therefore, Shudras are the only people in the Hindu society who don't have rights are freedom. However, they would have freedom and rights after they get the right to use a higher Varna.
While this practice is perfectly acceptable in Hinduism, it looks like exploitation to others. Therefore, Shudras are encouraged to exert their rights. Such Shudras are called as untouchables.
Thus, Untouchables are Shudras who demand equal rights.