You can finish many sewing projects with a basic presser foot, but there are also a variety of special-use feet and attachments available for sewing machines. A specialty foot can make adding buttons and zippers much, much easier, or add that last extra bit of polish or embellishment to a finished garment.
This skillset will walk you through some of the more common specialty feet. Some machines will come with 3-5 extra feet at purchase, and some do not, but all of these attachments are available from Amazon or directly from your machine manufacturer for a fairly low cost. Simpler feet will set you back about $5, and the most advanced ones can cost up to $20. You can also purchase specialty feet from your local sewing store, but watch out! The prices tend to be higher there.