What is a Portfolio? 

A portfolio is a collection of work that exhibits a student's achievement across a given period of time. 

Three Key Characteristics of Portfolio Assessment

Portfolios Promote Student Ownership
       One  of the most important aspects of the portfolio is that it is a 
       selection of student work (that is, the student is responsible for 
       choosing and justifying the pieces to be included). 

Portfolios Promote Self Assessment
       Traditionally, as part of the portfolio, students are required to 
       examine, in writing, the particular strengths and weaknesses of their 
       own writing.

Portfolios Emphasize a Process, Rather than Simply a Finished Product
       Portfolios usually contain first drafts and revised versions of individual 
       writing assignments.