Not all positions require a portfolio of your past work, but nearly all applications can benefit by a demonstration of your abilities. Regardless of your field of practice, a collection of your work presented in a visual way is a compelling way to prove your skills.
In addition to whatever visuals you can include, providing some context is also valuable. Explaining the requirements or background of a project helps clarify the way in which you met those needs. Likewise, the results of your work also confirms that your efforts were effective.
Variety is an important part of any portfolio. Use diverse pieces to show your flexibility in:
Unless otherwise specified, including 3-5 pieces of work is a good amount. More than that can be excessive, and including less doesn't establish much. Also: be sure to only include strong pieces - you certainly want to take a "quality over quantity" approach. A poor example can detract from the perception of your abilities, and diminish your whole portfolio.