A Baluster is commonly found on a staircase - they are the spindled, often wooden, forms that support the handrail going up the stairs. On ornate buildings they can also be found supporting parapets. In a series like these they are described collectively as a 'Balustrade'.
The primary purpose of a Baluster is support although a lot of emphasis can be placed on the aesthetic of the design; indeed, fashions and styles of Balustrades have changed throughout the ages.
Balusters are often placed close to one another for reasons of appearance. Most staircases do not require nearly as many Balusters as they possess but placing them far apart is not pleasing on the eye.