Conscience means moral sense, ethical feelings or one’s sense of right and wrong.
- His conscience wouldn’t let him steal.
Note that conscience is spelled with science at the end.
Conscience is a noun; conscious is an adjective. It is an inward alertness or awareness.
- Is she conscious yet?
- Were you conscious of his hostility? (= Were you aware of his hostility?)
- He was not conscious when they took him to hospital.
Consciousness is the noun form of conscious.
- The accident victim never lost his consciousness.
Conscientious is the adjective form of conscience.
A conscientious worker arrives on time and does his duty with utmost sincerity.