A semiconductor is a material (most often an element like silicon, but sometimes a compound like galena, used in early diodes) which is intermediate in its ability to carry current between an insulator (e.g. most plastics) and a conductor (e.g. most metals).
Semiconductors are very important to modern electronics because the addition of trace amounts of dopants (other elements) can render them into materials which have an excess of electrons (so-called 'N'-type semiconducors) or holes (the absence of an electron, so-called 'P'-type semiconducors). Joining together blocks of P- and N-type semiconductors in the appropriate configurations can produce a wide range of devices, including diodes and transistors.