If you get a rotary cutter, be sure to get a cutting mat to go with it.
The blades are like round razor blades - they need to go in some sort of holder/handle in order to use them. Usually a new handle/cutter has a blade already installed on it and the package says something like "45 mm Olfa rotary cutter".
One can "free hand" cut with a rotary cutter on the mat - but most people buy an acrylic/plastic ruler that is about 1/8 inch thick to go with the cutter and mat to help them make straight cuts.
The "best" ruler and "best" mat are whole other areas of discussion.
If you are just starting out, you might consider getting this set-up from WalMart.
http://www.walmart.com/ip/Fiskars-Ro...g-Set/12335930
Some people turn up their noses at Fiskars products, but I find them satisfactory.