Tarnish forms on copper, brass, silver, aluminum and other metals whose surface reacts to various chemicals in the atmosphere.  It normally has a bronze colour and can often be easily removed with a silver polishing cloth or chemical solution.  Tarnish does not cause any damage to the metal.

As the purity of silver decreases, the probability of tarnishing increases.  Silver itself is not particularly prone to tarnishing at normal temperatures, however other metals which are used in the alloy such as copper are, hence they cause this reaction.