If you plan on being a professional photographer you should keep examples of your work for past and future reference. In photography a portfolio is much like your resume. It contains past work you have done and it is available for potential clients to see. When putting together a portfolio most people select photographs they think are creative; artistic statements that deserve to be seen. They choose their best and sometimes favorite work in hope of impressing prospective clients enough into hiring them.
If you are applying for a job as a fashion photographer you should have some examples of fashion photography in your portfolio and sports photos for a sports job, etc. If you are applying for a general photography position it's a good idea to show how versatile you are by having a good assortment of photos available. Make sure you keep your portfolio in good shape and change it if you are having no success with it.
Many photographers also have an online portfolio to showcase their digital art. There are many free storage places for this. Another option is to create a professional website.
You can keep portfolios in various photo holding items such as albums or cases. These are available in various styles, materials, shapes, sizes and colors.
They can range in price from about $20 to over $100.