A portfolio is a collection of work that showcases your abilities. The purpose of a portfolio is to help people (potential clients, generally) to understand what you can do. Ultimately its purpose is to show that you are a competent professional.
Examples:
http://jessingrassellino.com/portfolio/arrow-up-right
http://www.satisfice.com/rst-appendices.pdfarrow-up-right
Last updated 5 years ago