Vector is an object with magnitude and direction. Vector product has lot of application in solving directed graphs.

In Euclidean space, a Euclidean vector is a geometrical object that possesses both a magnitude and a direction. A vector can be pictured as an arrow. Its magnitude is its length, and its direction is the direction the arrow points. The magnitude of a vector A is denoted by ||A||. 

Dot product of two Euclidean vectors A and B is defined by
AB=ABcosθ,where θ is the angle between A and B.
We can compute (determine) the angle between two vectors, given their coordinates:
cosθ=ABAB

Note the sign of this expression depends only on the angle's cosine, therefore the dot product is

  1. <0 if the angle is obtuse,
  2. >0 if the angle is acute,
  3. =0 if the a and b are orthogonal.
AB
Cross product of a vector is an orthogonal vector. 

http://betterexplained.com/articles/vector-calculus-understanding-the-dot-product/