Solder is an metal alloy used to combine two pieces of metals. Solder comes mostly in a shape of a wire on a spool. Solder comes in many different diameter's which depends on what are you soldering. Solder is an metal alloy which is combined with tin and lead. There are different types of solder which have different amount's of tin and lead. One of the different type of solder is 60/40 solder which means there is 60% tin and 40% lead.
Also there are solders which have a flux core in the center.