As with other Pseudo-Random Number Generator (PRNG) algorithms like the
Mersenne Twister (see Math::Random::MT), this algorithm is designed to
take some seed information and produce seemingly random results as output.
However, ISAAC (Indirection, Shift, Accumulate, Add, and Count) has
different goals than these commonly used algorithms. In particular, it's
really fast - on average, it requires only 18.75 machine cycles to
generate a 32-bit value. This makes it suitable for applications where a
significant amount of random data needs to be produced quickly, such
solving using the Monte Carlo method or for games.
The results are uniformly distributed, unbiased, and unpredictable unless
you know the seed. The algorithm was published by Bob Jenkins in the late
90s and despite the best efforts of many security researchers, no feasible
attacks have been found to date.