  A quantum communication channel can be put to many uses: it can transmit
classical information, private classical information, or quantum information.
It can be used alone, with shared entanglement, or together with other
channels. For each of these settings there is a capacity that quantifies a
channel's potential for communication. In this short review, I summarize what
is known about the various capacities of a quantum channel, including a
discussion of the relevant additivity questions. I also give some indication of
potentially interesting directions for future research.
