What is the dot product used for? code example

Example: dot product

Let a be a vector with coordinates (a1, a2, a3)
Let b be a vector with coordinates (b1, b2, b3)

Dot Product: 
 •  = a1*b1 + a2*b2 + a3*b3

a • b = |a||b|cosθ, where θ is the angle between the two vectors