Solder is a metal or alloy that melts at a low temperature. There are two types of solder; soft solder and hard solder. Soft solder melts easily with soldering irons and is used for electronics and electrical work. Hard solder melts at a higher temperature with a torch. Using solder is called soldering.
There are two main types of soft solder; lead solder and lead-free solder. Lead solders have about 60% (or 63%) tin and 40% (or 37%) lead in them. They are toxic because they have lead in them. They melt at around 185°C. Lead solder is cheap, so it used to be popular. In plumbing, a 50% tin and 50% lead mixture was used.