Empathy is a feeling of concern provoked by observing the suffering of another.
Compassion conflates this emotion with action (or beneficial inaction) appropriate to mitigating the perceived distress.
Pity may result from empathy and may resemble compassion, but it is absent of the requirement to acknowledge the sufferer as an equal.
Because of this, the works of pity are always sullied to some degree by its remote, subtly contemptuous nature. Whereas pity emphasises the distance between us, compassion dissolves it.