As defined by SMI:  'Extension springs are springs which absorb and store energy by offering resistance to a pulling force.  Various types of ends are used to attach the extension spring to the source of the force.'

Wire types are also quite varied.  Material types can be anything from music wire to titanium to beryllium copper.  Shapes differ from round, ovoid, square, rectangular, and stranded wire.

Ends on an extension spring are generally designed specifically for the application, but there are several basic types.  Click here for some examples.