Quantum Mechanics (QM) is the underlying physical theory of the very small; it has several highly bizarre features, including a notion of parallel worlds. Nevertheless, it is the direct source of the most exacting and precise measurements and predictions ever made, and is the basis for much of our understanding of the world around us.
Quantum Computing (QC) is the use of QM to perform (a bizarre kind of) parallel computation. This will be treated in a separate set of notes.