A part of Concurrency concepts.
[Someone have some URLs that discuss what threads are in general?]
See John Ousterhout's homepage for the Powerpoint slides of his talk Why Threads Are A Bad Idea (for most purposes).
See these pages for more info:
Threading is "fault-prone, deadlock-prone, priority-inversion-prone". Lock (mis)management makes it poor in scaling. It's hard to reason about thread behavior [justify].
DKF wrote in the comp.lang.tcl newsgroup: "When I deal with threading, I work on the principle that the hardware and OS are my enemies and hate me, and so I code accordingly. It probably doesn't make for a maximally-efficient program, but it does reduce debugging pain."