There are three basic types of picture frame mats you can buy. Each offer different levels of protection for your framed item.
You may need museum quality mats for some things, but more often standard mats will get the job done.
Wood (which paper is made of as you know) contains acids which can, over time, stain the items that are in contact with it.
The difference among the 3 types of mat board is the amount of paper used and the "threat" it poses to your work.
Professional framers feel very strongly about using the more expensive higher quality mat board in most cases.
I am not saying they are wrong by any means, but for people just starting to learn the art of matting, I don't think that is necessary.
Below is a simple breakdown of each. Learn about all 3 and their pros and cons. As you develop your skill and take on matting more expensive or sentimental items you will be ready to upgrade to the appropriate type of mat board.
This is the cheapest of the three options when it comes to picture frame mats and really all you will need for most of your projects. Standard matboard is made mostly out of paper, but buffered with a chemical that counter-acts the acidity. It is typically listed as 97% acid-free.
It could potentially stain your items, but would take a long time (in most cases -decades) before any staining or discoloring occurs.
Is made from 100% cotton (which is acid-free) but has a buffered paper cover on top of it to give it its color. The chance of the acid on the top layer working its way through the cotton board and to your artwork is very small and would take a very long time.. Conservation matboard is more expensive than standard matboard, but it will protect the framed work for, oh about 100 years.
This type of mat is used in museums and for archival documents. It is totally acid-free and will protect the framed items "forever". Because of that it is also the most expensive of the three.
There are more options out there for picture frame mats, like leather, suede, and other fabrics, but for our purpose here they probably don't fit in. I may add something in the future on more expensive types of matting.