My portfolio includes awards, credentials, sample student assignments, assessments, record keeping, lesson plans, and photos.  The modern portfolio is digital.  Some would say that portfolios are kept online on employment sites like LinkedIn.  I can imagine that in the near future, perspective teachers will scan their iPhones and employers will view a commercial or video presentation.

Portfolios serve as a record of your skills and can demonstrate your professional growth over time.  It is proof of what you do.  It is accountability.