Solder (pronounced "sol-der", US "sod-er") is a metal alloy which is used in R/C to bond certain electrical components together. It is applied in a molten state by use of a soldering iron, a device which heats the parts being soldered to a sufficient temperature to allow the solder to melt and flow over them. A good example of the use of solder is joining wires to an electric motor's power terminals. On a smaller scale, solder is used to anchor and connect electronic components on printed circuit boards of the type found in receivers, servos and speed controls.
Solder used in electronic work is comprised of 60% lead and 40% tin. A rosin-based flux is incorporated into the solder; this is a lubricant that further aids the solder in flowing over the work. The smoke and distinctive odor of this type of solder comes from the heating of the flux. This type of solder is identified on the label as "60/40 rosin core." Flux is also sold separately and can be brushed on the work to further aid in lubricating.
There are other blends of solder and other types of flux, but these should never be used in electronic soldering. 40/60 solder and silver solder have too high a melting point which could lead to component damage. Silver solder, once melted, requires an even higher temperature to re-melt it. Under no circumstances should acid flux solder of the type used in plumbing work be used in R/C or any other electronic work. This type of flux is highly corrosive and designed to etch the metals it is intended to join. This same acid will destroy electrical and electronic gear if used.