Ruby Programming/Reference/Objects/IO/Fiber

A Fiber is a unit of concurrency (basically a manually controlled thread). It is a new construct in 1.9

1.8 basically used green threads similar, to fibers, but would pre-empt them, which 1.9 does not do.

See its description.

Several useful things have been built using fibers, like neverblock, the revactor gem, et al.