A cell of the lymphoid lineage in the immune system.
Unlike T-cells that recognize specific peptide antigens presented in the context of MHC molecules, NK cells recognize cells that have lost expression of MHC molecules on their surface, and kill those cells.
It all makes sense, if you know how T-cells work, and how evil viruses work.
T-cells protect the body in a number of ways. One of the ways it protects the body against viruses, for example, is to identify virally infected cells and kill them. This is done by CD8 expressing T-cells. They don't touch cells expressing MHC molecules bearing peptides derived from the body itself, but T-cells can recognize cells that express MHC molecules bearing foreign peptides, such as those from viruses.
This works out remarkably well, but viruses have evolved just as much, if not more than humans (and other animals). Some viruses, chiefly large DNA viruses, have genes that specifically inhibit the processing and presentation of viral peptides by the MHC molecules. Clever bastards!. But this is where NK cells come in. Some viruses are able to evade T-cells by simply shutting down MHC synthesis. No MHC, no recognition by T-cells. But, the NK cell can recognize this lack of MHC, and *blammo!*, the NK cell takes out the virally infected cell.
As an additional note, because of other aspects of NK cell biology, it seems that NK cells may be more evolutionarily ancient than T-cells, and that maybe T-cells arose from NK cells during evolution.