Pure silicon is, in fact, not very useful. It conducts only slightly at room temperature, which leaves you very little room to control it. So we deliberately mix in a trace amount of another element. This is called doping.

Silicon has four valence electrons, and each atom joins hands with its neighbors to form a crystal. Mix in phosphorus, which has five valence electrons, and four of them are used for joining hands while one is left over. That spare electron is not tied down by any bond, so it is free to move. Because there is a surplus of electrons (negative), this is called an n-type semiconductor.

Mix in boron, which has three valence electrons, and you are one hand short instead. This state — "a place where an electron ought to sit is empty" — is called a hole. A hole shifts position as a neighboring electron moves into it, so it behaves exactly as if a positively charged particle were moving. Because the hole (positive) plays the leading role, this is called a p-type semiconductor.

The amount of doping is tiny — on the order of a few to a few thousand impurity atoms for every hundred million silicon atoms. That alone changes the conductivity by orders of magnitude. Put the other way around, if unintended impurities get in at that same concentration, the characteristics are ruined — which is exactly why semiconductor fabs need such extreme cleanrooms.