Quantum mechanics is the "small stuff" of physics. It is the study of "tiny stuff" like subatomic particles and atoms. Quantum mechanics includes weak nuclear force, strong nuclear force, and electromagnetism. These three forces in quantum mechanics cannot be analyzed in terms of general relativity.
Because quantum mechanics and general relativity cannot be combined, the string theory is thought to be the key to unifying them. Just like Newton unified gravity to explain it on all levels, using the string theory is hoped to allow quantum mechanics and general relativity to combine in one equation that can explain everything in the universe.