A gist is basically a mini-git repo. There is a revision history and they are attached to your github page. You can also folk or edit them and there is a permalink to each revision.
There is programmatic access to gists via the github API.
This might help clarify: An example of using gist.io: http://gist.io/3135754 The underlying gist: https://gist.github.com/3135754