[quercusa]

There are true RNGs that use a physical source of randomness such as resistor noise [0] feeding into a comparator. Most of the things like mouse movements can be random(ized) but tend to be really slow (few random bits per second).

Psuedo-RNGs typically use an algorithm (such as a hash) which is very hard to reverse. The next value is deterministic and easy to predict if you know the algorithm and input values, but effectively impossible (i.e., 'random') if not.

[0] https://news.ycombinator.com/item?id=27447766

[lann]

> Psuedo-RNGs typically use an algorithm (such as a hash) which is very hard to reverse.

Note that this is only necessarily a property of Cryptographically Secure PRNGs. Some PRNGs are not designed to be difficult to predict at all.