[kdelok]

It's because most of the randomness used by software is actually pseudorandom. What that means is that you actually use a defined sequence. The sequence has behaviour that's close enough to what you'd get if you were picking random samples from a distribution for the desired application.

The key difference is that it's reproducible and that if you have insight into the parameters of the sequence (e.g. the seed and the current position in the sequence), you can predict the results. That's why people often get upset when people use these pseudorandom number generators for security purposes.

The seed is a value that is used to generate the sequence. If you use the same seed, you get the same sequence.