Web designer portfolios are fairly easy to come up with when you are first starting out. As you move up in the field it can be a bit more challenging to select the Websites that best represent the full range of your abilities, but that's exactly what will help insure that you are considered seriously for jobs in the field.
Showing what you are capable of is the ultimate goal of a portfolio. Since you can never know ahead of time what people may be looking for, it is a good idea to include a wide range of work. For example, if you have created a Website that has a forum section and one that has a great flash opening sequence, you'll want to include both so that people are aware of what you can do.
What happens if you have a Website that showcases a particular skill, but you don't want to include it in your portfolio for one reason or another? Whatever doesn't fit in with other work can be outlined on paper--either in a profile or on a resume.