Question: What is a portfolio, and what should it consist of? Please include details such as the format, use of, number of items, types of items, etc.

Answer: 
A portfolio should sum up the work you have done and the impression you are trying to give. Generally speaking ,if you are a web designer, a portfolio fo print work would be less relevant than your web design work so you would promote your web design work over your print work.

Think of it like a colourful CV/resume. You would taylor it to the job you are applying for. If you are less experienced then you would show the aspects of your portfolio that reflect relevant knowledge that can be applied to the field you are aiming at. With more experience in a specific field you can then select the best work of that field alone.
A portfolio is a place where one includes their best work to show it off. Portfolio pages help in the selection criteria for business services. [Here](http://www.slideshare.net/webdesigntips/what-is-portfolio-website-design) is a slide that covers almost all about what a portfolio is and what it requires, including the importance, etc.