What type of mat will protect your art/photos long-term? You have choices, so be aware of what the mat is made of; some materials can actually damage your artwork.
There are three typical types of mats you can get: paper mats, Alfa mats, or rag mats. The difference between them is the acid.
Paper mats, made of paper pulp, are the most inexpensive and used with a lot of frames that come with the mat. These mats have a lot of acid in them that causes the edges to turn brown. Eventually this can ruin the art as the acid leaks into it. You can spot a white paper mat by looking at the beveled edge where it meets the artwork. It will not look pure white.
Alpha-cellulose mats are made of wood pulp chemically treated to eliminate acids that can decay artwork.
A rag mat is a naturally acid-free mat board made from cotton. These mats have no acid, and the cut edges stay pure white forever. Rag mats, used by museums, are a popular choice for keeping artwork safe.
Go for a rag or alpha mat for artwork you want to protect forever. If you are getting a mat from a professional framer, make sure it has no acid in it.