A portfolio is simply a collection of your writing that’s often used to demonstrate your abilities. Students in creative writing workshops often prepare a portfolio as an end-of-term project. Graduate programs in creative writing may ask for a portfolio as part of your application materials, as do some grants and fellowships. You might also see requests for a "writing sample," which—in the case of applications—is essentially the same thing.
When putting together a portfolio, always look at application guidelines. Many institutions are very clear about what they want to see and will provide details regarding genre, page count, and other specifications. Follow those guidelines. The committee will be looking for what the application stipulates.
Beyond the guidelines—or when guidelines are open—consider the purpose of the portfolio when selecting materials. If you’re applying for a creative writing program, you might want to include pieces that highlight different skills and demonstrate your literary merit. If you’re applying for a fellowship that has a focus on writing and the environment, you’ll want to pull from work that prominently features that particular theme. Grants geared toward poets probably won’t be interested in your non-fiction essays or screenplays, no matter how well they’re written.
Be selective in what you include. Tossing in everything you’ve written recently and letting them sort it out isn’t a good approach, even if the page limit is generous. Include only your strongest work. If appropriate to the portfolio’s purpose, show your range as a writer. Three stories with different first person narrators all written with a similar voice may indicate to a committee that you’re only willing—or able—to write in this one way. You may not want to project that image. At the same time, a portfolio that’s all over the place—an experimental micro-fiction, a poem that mimics the choices in William Carlos Williams’ "Red Wheelbarrow," and a story written in the style of James Joyce—may indicate that you aren’t sure who you are as a writer.
Show your selections to another writer or a good reader. What kind of writer does the work suggest? You know everything you’ve written and probably think about yourself as a writer in terms of that whole body of work. Having someone look at the selections you’re considering as a portfolio may help you understand more about the image you’re creating for yourself.
Don’t feel compelled to fill your portfolio to maximum page limits or to stuff it with everything you’ve ever written. If you squeeze in a mediocre five-page short story just to meet a page count, you’re giving the committee an opportunity to see your less compelling work. Still, submitting three pages when you have the opportunity to show fifty sends a message, too. You don’t want to give the impression that you haven’t written very much.
Always do a final edit before you send out your portfolio. Typos, misspellings, and other careless mistakes reflect poorly on your attention to detail. That is, after all, one of the writer’s most important tools.