Solder is a metallic alloy used to join pieces of metal, most commonly electrical or electronic circuit components, though different types of solder are used in a wide variety of applications, plumbing being another common one.
Electrical solder is composed of tin and lead, usually about 60% tin and 40% lead, which is close to the ideal composition for a tin-lead mixture. At 62% tin and 38% lead, solder is a eutectic alloy; it melts at the lowest possible temperature and changes phase (i.e. melts and freezes) uniformly. Tin is more expensive than lead, however, and cheaper solders have more lead than tin, which makes them more difficult to work with.
Since lead is a very toxic substance (one of the reasons it's no longer used in gasoline), soldering should always be done in a well-ventilated area to minimize inhalation of the poisonous vapors. Lead-free solders are available, but they can be very expensive.