A toolkit is a collection of widgets for use in a windowing system.

What this means is that a programmer uses a toolkit to make GUI programs. The toolkit is responsible for determining how the elements of the GUI look and feel. This is a Good Thing, because:
  • It makes programming easier, since the programmer doesn't have to reinvent the wheel (by writing code to handle making buttons and so forth)
  • It makes programs consistent. Programs that use the same toolkit will have controls that look and operate the same.
Some Operating Systems are tied to a single toolkit, such as Microsoft Windows, BeOS, and MacOS. Others have multiple toolkits: basically anything running X (Unix, Linux, BSD, etc.). Each approach has its advantages. Having a single toolkit means that all programs will have the same look and feel, but this sacrifices flexibility and configurability.