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 collection of your work. It's used to demonstrate your capabilities, usually as part of a job-seeking process. It should have your best work from whatever it is you do.

The rest is left as an exercise for the student.