No source provides completely random numbers but some sources are more random than others:

intervals between keystrokes on keyboard
rapidly changing areas of RAM
network packets arrival times
disk seek times (speed variations are caused by air turbulence)
hardware random number generators (unstable free-running oscillators, radioactive decay, thermal noise from an amplified diode)

Pseudorandom numbers - if you know what number the computer just picked, you can predict the next number.

Congruential generators

lame but fast; so they are used often :-)
algorithm:
1. take the last random number (Nj)
2. multiply it by something (A)
3. add something (B)
4. take the remainder when you divide by something else ©
  
  Nj+1 = ANj + B(mod C)
Every random number is derived by the previous random number except for the first, which comes from you. This first number is called the seed of the random number generator, and you provide it to Perl (and C) with srand().

Source: