Randomness is determined by the entropy in a system. A computer, with no external stimuli, cannot generate randomness, only pseudorandomness. That is, it may to all intents and purposes appear random, but it follows a predictable pattern. In many cases, by observing a sequence of pseudorandom data, one can determine what will follow. Thus it obviously can't be random, because randomness must be unpredictable. The most common use of randomness in computing nowadays is cryptography, which requires that the randomness is, as far as possible, entirely unpredictable; otherwise the potential exists to determine what the content of the encrypted data is, or to forge an encrypted message.
So mechanisms exist to introduce entropy into the equation. Entropy in this context means unpredictability -- it is the amount of "unguessable" data which has been fed into a system. Computers often generate entropy by using the timing of unpredictable external stimuli, such as keystrokes or network activity. Feed these into the pseudorandom number generator, and what comes out is significantly more random (i.e. less predictable) than the basic pseudorandom number generator. But it is (theoretically) possible to influence the random output by controlling the entropy going in, which must be a consideration in any case where true randomness is needed.
Electronic circuits needing randomness sometimes use semiconductor junctions known for their noisy behaviour (usually an undesirable characteristic!), and feed the entropy from this noise into the generator. But these methods are prone to producing biased results, which may be unpredictable but not statistically well-distributed. They can also be affected by external sources such as electrical interference, which again makes them less suitable where a high degree of randomness is essential.
If you want
real randomness, you want one of these:
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This device works, in simple terms, by firing individual photons (light) at a partially reflective surface. About half the light will be reflected into one sensor, the rest will pass through the surface into a second sensor. But an individual photon cannot do both; it will either be reflected or it won't. Now, quantum theory tells us that it is not possible to predict in this case which individual photons will go which way. We know that about half will go each way, but where each individual photon ends up cannot be predicted.
This is where we finally meet true randomness. Randomness, of the true variety, is very important to quantum physics. For instance, Heisenberg's Undertainty Principle tells us that we cannot predict both the position and the momentum of an electron at the same time. In other words, you can calculate one property, but the other is unpredictable, i.e. truly random. So, in the quantum world, randomness really does exist. Einstein struggled with this concept for a large portion of his career, when the new generation of physicists developed the field of science he had largely created. He could not accept that there came a point at which things could not be determined mathematically, hence his famous (paraphrased) quote, "God does not play dice with the universe." However, it seems, God
does play dice with the universe, and some things truly cannot be predicted.
