There are a couple types of circular hooks -- one has a stop on the end used for tunsian crochet. The other has a hook on each end used primarily for crochnit/double ended crochet.
I prefer the double ended because you use them for both methods. Plus they come longer -- 40 inches or so.
Here's a pic on my old crochet blog that I abandoned
http://alovelycrocheter.blogspot.com...-circular.html