Pseudo-Random Number Generator - technical definition

A random number generator creates a sequence of randomly distributed numbers. A Pseudo-Random Number Generator creates random numbers as well, but it will create the same sequence of numbers repeatedly. Many algorithms have been developed in an attempt to produce truly random sequences of numbers, with the goal of making it theoretically impossible to predict the next number in the sequence, based on the numbers up to a given point. Unfortunately, the very existence of an algorithm that calculates this number means that the next digit can be predicted.

For all real applications, PRNGs are considered to be sufficient. PRNGs play a role in encryption schemes that use random numbers as part of the encryption process. It has been shown that weak, predictable PRNGs make the encryption less secure and therefore crackable.