Tinned copper wire is copper wire coated with a thin, electroplated layer of tin. This type of wire may be composed of a single tin-coated copper cable or many individually tinned strands of copper wire bound together. It is available in insulated versions and in uninsulated, or "buss wire," versions.
The Reason for Tin
Tinning was originally developed to protect insulated copper wire from corrosive chemicals -- primarily sulfur -- released by wire-insulating materials. While no longer used in most wire insulation, these materials do sometimes show up in cable produced for a few high-environmental-stress industries, including waste-water treatment and paper milling. Tinning makes it easier to solder copper wire because it readily bonds to solder, which is itself largely made of tin. Tinned copper wire also suffers less corrosion at high temperatures than untinned copper.