Work samples are the actual pieces of work housed within the portfolio. Work samples can include photographs, designs, film clips, advertising campaigns, air-checks and many other types of work.
Some industries expect a full line of pieces, such as a fashion design line or fleshed out advertising campaign. Others prefer a few samples that are indicative of the finished piece, such as film clips or journalism abstracts. It is best to research the specific industry to learn the latest standards regarding portfolios and portfolio presentation.