A method for generating values of a continuous random variable for use in a simulation. Suppose that the random variable X, which takes values in the interval (a, b), has probability density function f. Denote the maximum value of f(x) by M. Let u and ν be two random numbers uniformly distributed in the interval (0, 1). Write r=a+(b−a)u and s=Mν, so that r and s are uniformly distributed on (a, b) and (0, M), respectively. Calculate f(r). If f(r)>s then r is accepted as a value of X. Otherwise, it is rejected and a new pair of values is taken for u and ν.

Acceptance–rejection algorithm. Uniform random numbers are generated in the intervals (a, b) and (0, M). If the point generated lies between the graph of f(x) and the x-axis, then the value of X is accepted.

http://playingwithr.blogspot.co.uk/2011/06/ R code and demonstration.