rocker: amount of bend in the board from tip to tail (ie: if you set the board flat on the ground, rocker is measured by the distance from the ground to the tip and/or tail of the board)
-Types of rocker: 3-stage (kinks in the rocker, generally created by a flat spot in the middle of the board) and continuous (smooth curve all the way through). There have been endless discussions on the advantages of different rocker types, so instead of summarizing them for you, I'll trust your ability to do a search on the message board.
length: self explanatory
width: width of board at center
sidecut: the general shape of the board, mainly determined by the tip/tail width compared with the width in the middle.
rail: the edge of the board on the sides (where the top sheet meets the bottom sheet). Types: cupped rails (turned down to provide greater edge hold and tracking, but are less forgiving), soft/beveled rails (slightly rounded-off to make the board more forgiving, but not quite as "edgy"), different in-betweens, and the amazing "variable rail" (cupped at the tip and tail, rounded in the middle- In my opinion, the best of both worlds. eg: many DoubleUp shapes have this feature)
channels, molded fins, etc: What are all those crazy things on the bottom of my board?? They help to direct water flow, affecting the edging characteristics of a board. Also, the way a board lands is influenced. Eg: If you have channels throughout the entire length of the board, water is only displaced in two directions when the board hits the water, resulting in harder landings. Consequently, a featureless bottom in the center, or one with a center "beam" will help to displace more water in all directions.
tip/tail shape: This is one of the least important factors in my opinion, and variation primarily occurs to make boards stand out from one another.
Oh, and I almost forgot...
graphics: hands down, the most important part of a wakeboard.