dotz..

• Hitting one of the large portions of each of the numbered sections, traditionally coloured black and white, scores precisely the points value of that section.
  
• Hitting the thin outer portions of these sections, coloured red and green, scores double the points value of that section.
  
• Hitting the thin inner portions of these sections, roughly halfway between the outer wire and the central circle and again coloured red or green, scores triple (or 3x) the points value of that section.
  
• The central circle is divided into a green outer ring worth 25 points (known as "outer", "outer bull", or "iris") and a red inner circle (usually known as "bull", "inner bull" or "double bull"), worth 50 points. The term "bullseye" can mean either the whole central part of the board or just the inner red section. The term "bull's ring" usually means just the green outer ring.

source: http://en.wikipedia.org/wiki/Darts