Balusters or pickets are the rail components used to provide infill for deck guard rails. IRC code prohibits guard rails from having any openings that would allow a 4" sphere to pass through. This rule produces the need for an attractive and economical repeated system for filling in the openings between rail posts and rail top and bottom members. 
Because of the spacing requirement you will need a lot of pickets or balusters even for a short length of rail. Balusters will certainly be the most expensive rail component for your project. Years ago most deck builders used wood 2x2's nailed to the deck rim and top rail to meet the requirement. Some builders even used lattice. Thankfully this practice has mostly disappeared.
Many creative deck builders have crafted unique rails using conventional wood parts in different applications such as using horizontal and vertical members to create a grid or setting 2x2's at angles to form a sunburst pattern. Along the way may professional deck builders started installing copper tubing into drilled out holes along the top and bottom rail. This was an interesting look but required substantail labor.
Today you can find a wide variety of balusters and pickets on the market. Many metal balusters can be purchased in assorted colors, shapes, and sizes. Fortress Railings offers tempered pureview glass balusters as well as stylized powder coated metal. Fortress Railings offers connectors that allow easy installation without drilling.
Pickets and balusters can be installed to the face of rails or in between the top and bottom rails. Either way you need to make sure they are properly attached to prevent anyone from falling over or through the rail.
