Anellini are tiny little pasta rings used as a soup pasta. This little pasta is cousin to a larger version called Anelli which is famous for the main ingredient in SpaghettiOs. Anellini can also be used in salads as well as some baked pasta dishes.
Anellini is not as common as some other soup pastas such as orzo. Look for it in the pasta isle of any well-stocked grocery store or Italian market. If you can't find it locally then you can order it online at Amazon.com: Anellini.
If you don't have Anellini then you can substitute equal amounts of another soup pasta such as: