A portfolio, here, is a collection of written work, related or not, presented as well as it can be, by a student for the purposes of showing, well, the best that the student can do at a given time.
Online, portfolios allow wide latitude in individual expression, and can contain a wide variety of kinds of work: research papers, essays, weblog entries, paragraphs, journal entries, summaries or creative work. There is a kind of dynamic tension at all moments with weblog portfolios: on the one hand, they should have visible, from the first screen, all the best of the student's work, properly formatted, edited, looking crisp and nice (defined more carefully below) and properly linked. On the other, the weblog is a dynamic thing, receiving the latest of the student's work, and pushing older stuff down and out of sight.