Ruth has already described it in great detail.
The Crasy Trio is easy to use thanks to its three needles, and the flexible middle sections (similar to a mini circular needle) allow you to secure the knitting project well so nothing falls off. I learned to knit socks with it; before that, using the five needles of a conventional set of double-pointed needles was too fiddly and bulky for me.
For many double-pointed knitters, however, the Crasy Trio is too cumbersome—I think everyone has to try it out for themselves.