Definition--Students collect of their best writing once, twice, or three times during a term. Some teachers set limits on the kinds of papers; some require a certain number of pages.

Promises (Rationale)
As a teacher of comp, I struggled with ways to make practices match my preaching--I encouraged revision in writing process, but grading practices seemed to cut off revision prematurely. Then I discovered portfolios. They

encourage revision through the entire time allotted to the portfolio or even through the entire semester,
promote multiple readings of work-in-progress to shape writing effectively for audience and purpose,
encourage effective peer review (peers might see the same paper several times),
encourage students to ask for teacher intervention more frequently in process,
encourage self evaluation (students must choose their best pieces),
offer safe opportunities to experiment, and
give students time and space to develop ideas.

Problems

Students wait until the last minute to begin writing.
Solution: Assign regular "due" dates or regularly scheduled workshops at which drafts are required and checked.
Takes too much time.
No Solution! When the final portfolios come in, it's like taking two or three sets of papers home at once. So portfolios definitely will not save time. But I spend much less time on intervention drafts than I used to on final papers, and if I've seen most of the final portfolio pieces in draft, I don't have to spend as much time on them as I would on a brand new piece.
If students don't take the initiative to ask for intervention, some students can go for a long time without feedback.
Solution: Regular workshops will give all students frequent opportunities for peer review.
Another Solution: Occasionally, I require an intervention draft from everyone.
I spend so much time on intervention drafts that I have doubled my total grading time.
Solution: Comment on only the most significant feature of a draft (the element that will result in the most significant revisions).
Students do only superficial revisions.
Solution: Suggest students do as much writing as possible on a computer so that revisions don't require a lot of re-typing. Since I've switched to a computer classroom, I haven't had this problem. You could also require revisions of genre, audience, purpose, etc., to encourage students to make global revisions.