One of my colleague's asked what is fill rule,

"The 'fill-rule' property indicates the algorithm which is to be used to determine what parts of the canvas are included inside the shape. For a simple, non-intersecting path, it is intuitively clear what region lies "inside"; however, for a more complex path, such as a path that intersects itself or where one subpath encloses another, the interpretation of "inside" is not so obvious."  - From the SVG Spec January 14

the following set of pictures will help understand the need for specifying the fill rule for self intersecting paths,

Fill Rule: Even-Odd

Fill Rule: Non Zero