(A1) What is "World Building"
World building refers to the art of creating an imaginary world. Although called "world" building, the scale of creation can vary greatly. At times it can be as small as a town, or as large as a universe. The world that is made typically depends on the scale of the story and how willing one is to explore it. A self-contained tale of a young boy growing into man hood, or other slice off life tales, can be contained within a small rural town, but tales involving conflicts between nations require national scale at a minimum.
(B1) How detailed should I be in my world?
As detailed as the story requires! In open ended RP's or stories you might need to decide on the daily comings and goings of the shop keeper on the edge of town, or the life of soldiers before the war, and so on. In small scale and self-contained stories or RP's, you might only needs one house and two people fully developed. the world should be built to accommodate the story.
It's best to break down your world into four parts;