What the hell is entropy?
In computing, entropy is the randomness collected by an operating system or application for use in cryptography or other uses that require random data. This randomness is often collected from hardware sources, either pre-existing ones such as mouse movements or specially provided randomness generators. A lack of entropy can have a negative impact on performance and security.
Have you ever tried to generate a
ssh key and ran out of entropy?
Your system maintains and refills entropy from various sources. Key
strokes, mouse movement, disk activity. Basically, everything that can be
measured in a non-deterministic way.
For example you can act completely retarded by collecting entropy with an accelerometer.
If you really wanna know how to deal with entropy nicely and how it evolved watch Randomness: how arc4random has grown since 1998 (OpenBSD). A short summary of how different operating systems deal with randomness can be found on Wikipedia. There are also some new interesting ideas for the Linux kernel see lwn.net.