The surface of the deck is made of decking also know as decking boards.  You can think of it as the skin of your deck.  The deck frame is like the skeleton that presents the mass and structure of the deck while the decking or decking boards presents a thin layer of covering for use.  Decking is structural in addition to being an aesthetic element for deck construction.  Decking must be able to support 40 lbs live load + 15 lbs dead load as it is supported between joists.
Most deckings are designed to be installed across 16" on center joists.  

For many years decking was primarily wood; people used redwood, cedar, pressure treated pine, and even some exotic woods.  Wood has some beautiful natural characteristics but also requires a lot of maintenance.  There are over a hundred different men made decking products on the market today.  Most are composed of composite decking:  GAF products and Azek products offer low-maintenance and a variety of appearances and color.

Wood decking will require you to stain your deck at least every year for the first several years to protect the surface from sun and moisture damage. When the decking is installed it is usually high in moisture content which can lead to cracks in the wood if it is allowed to dry too quickly.
Different decking materials have different installation requirements.  Wood decking is usually nailed or screwed to the joists for attachment.  Alternative decking products have a wide range of fastening systems but the most common is the hidden fastener system.

GAF Composite Decking / Correct Decking and Azek Composite Decking / Azek Trim Boards have this system which it is a channel on both outside edges of the board allowing the insert of polypropylene biscuit fasteners eliminating  facing nails or screws creating a nice and smooth surface. Your choice in decking is very important because it is the surface you will see, walk on, maintain, and have to live with for years to come.