Adobe's PDF format has beaten off the competition to become the de-facto portable document format. While Microsoft Word might be slightly more ubiquitous, it lacks PDF's ability to look exactly as the author intended on pretty much any platform you care to mention.
For designers creating a portfolio, this is a hugely important factor in an industry where presentation is just as important as the work on display. Users who have downloaded Adobe Reader software also have the ability to view PDFs inline without having to leave their browser, which means that PDF portfolios no longer have to be emailed but can live on a server somewhere. All you need to do is email the link.
The PDF format has quite a few tricks up its sleeve, such as the ability to incorporate complex interactivity, featuring buttons and links, and the ability to embed audio and video in addition to images. Using the easy-to-use Acrobat Professional application, it's possible to completely control how users see your document.
All you need is an application to create the initial PDF, although Acrobat Professional can create a PDF from a rag-bag of image files if necessary. On the Mac, pretty much any application that can output to a printer can generate a PDF via the print dialog box, and there are plenty of cross-platform applications that will create PDFs natively.
For a multi-page portfolio, Adobe InDesign seems a good choice, especially as it has the ability to create rudimentary interactive controls that can be embedded into the PDF. For a wider range of interactive controls, it's best to use Adobe Acrobat, but for simple navigation, InDesign is easy enough to use.
The final piece of the jigsaw is the powerful Adobe Acrobat Professional software, which comes with most versions of Creative Suite collections, and can be downloaded from Adobe's website for a 30-day trial.
Acrobat 8.0 Professional, which is part of the CS3 software, has some great new features including the ability to collate PDFs into a single file while maintaining the security settings of the individual files, but the functionality required for this tutorial has existed in the previous versions of Acrobat going back to CS1 and earlier.
All you need is a great design, and you'll soon be showing off your work in the most flexible and portable way possible.