Lumaconi is a type of shell pasta and the name translates literally to "large snail". The pasta shells are pretty large, slightly larger than a large cherry or grape tomato dry, but expand even larger when cooked. The pasta can be served stuffed using a pastry bag with a tip (time consuming) or simply with a nice sauce. The sauce will nestle in the grooves and hollow of the pasta.
Look for Lumaconi in a well-stocked grocery store or specialty food store. Garofalo is a reliable brand. World Market carries it sometimes. You can typically find it online at Amazon.com: Garofalo Lumaconi Pasta