The simple way of thinking:
Each component has a failure rate. It can be measured in MTBF (mean time between failure) (as @Nick T said) or Failure Rate. Reliability is a number which states the probability that a component would still being working after an amount of time. In general for eletronic components it follows an exponential distribution. You can use it to calculate the probability of a component still being functional after a given time.
For a circuit with no security systems (back-ups, redundant circuitry, etc) the overall reliability is the product of each reliability.
If you have a backup circuit, for example two redundant capacitors on the power supply, the probability that the capacitor bank is still working is the sum of the two probabilities.