Samui is the largest of the many islands scattered around the southwestern corner of the Gulf of Thailand. The island, whose residents used to make their living off of fishing and coconuts, has seen massive development as a major tourist site, probably second only to Phuket in popularity.
There aren't many good dive sites in the immediate area of Samui, but if you want to fly into the Gulf, Samui's award-winning airport is the closest one. But, more than that, many people choose to base themselves on Samui, rather than Koh Tao. Samui has better range of accommodations on offer, especially at the higher end. Then there's more restaurants to choose from, as well as non-diving activities, which can be important if you're traveling with non-diving friends or significant other.
See the Thailand for Visitor's Samui Guide for a complete run-down on all there is to see and do on Samui.