There are multiple types of glazing, or generally stated, glass options to use when framing a piece of art.
The most common type is your premium clear glass. Most of your photo and poster frames come with this basic option. Premium clear is ideal for decorative, inexpensive pieces with low light surroundings. There is also the non-glare option for the basic premium. Non-glare is a good option for the same art pieces in a high light situation such as by a window.
The next type is UV protective conservation glass. Again, these come in clear as well as non-glare. This particular type of glass has a protective coating that goes against the image and protects it against fading due to any type of lighting situations. This is the most generally recommended type for your art's protection.
Non-glare options noted above have a certain amount of opacity on it, closely related to the finish of frosted glass. This type is not suggested for images with sharp lines, or multiple matting layers.
Then there is museum quality glass. This is one of the most expensive forms of glass,for good reasons. Not only does it come with the UV coating, it also has a non reflective quality that is different from your non-glare options. Museum is clear with an additional coating that does not allow light to reflect, it appears as if there is not even glass on your piece.
Beyond glass glazing options there is also plexi glass/ plastic options. With the advancements in framing they now offer plexi with UV protection. Plexi is good for children's bedrooms, since it cannot shatter like regular glass. It is also a good option if you plan on shipping a framed piece. Down falls with plexi include the fact that it is extremely easy to scratch the surface. It is also very static prone, which makes it difficult to clean.
One other option worth mentioning is laminating. Lamination overlays come in a variety of surface finishes that include, satin matte, luster and canvas. This is a permanent, heat treated option that protects your art and significantly lightens the overall weight of your piece. This option is good for displaying maps, children's art, and very large pieces.
I would use caution when thinking about laminating a limited edition print or any other type of original artwork worth a substantial amount of money since laminating is a permanent process that can decrease the value of the art in the long run.