Glazing is the clear covering of the frame package. Usually glass, but there are options, each with advantages and disadvantages.
Regular window glass is the most commonly used glazing. Window glass is readily available and inexpensive. It is easy to clean. Glass is available at big-box home improvement stores either cut to standard sizes, or cut to order.
Ther are a couple of disadvantages to regular glass. The first is that it shatters, so it poses a potential danger to the work. It does need to be handled with care, as the edges are extremely sharp. It also has a slight blue-green color cast to it. Despite these drawbacks, regular glass remains the most used glazing.
Museum glass is a rather high-tech form of glass. It is optically clear, with no color cast. It is actually a laminate, with layers that filter out ultraviolet light (the main enemy of artworks), and reduce, or nearly eliminate reflections.
The drawback to museum glass is cost, which is something on the order of fives times more than regular glass.
Acrylic sheeting (commonly called plexiglass, which is a trademark), is often used for frame glazing. Acrylic is very much like glass in that is (fairly) readily available and relatively inexpensive.
The major advantage to using acrylic is that it does not shatter. This may be the only advantage. Acrylic is fairly easy to cut. However, it comes with a paper or plastic cover, which must be pulled off. This creates a lot of static, which attracts dust. This makes cleaning it a bit problematic.
Acrylic requires special cleaners, as ammonia will fog it. It is very easy to scratch, even while cleaning it. Don't use paper towels! It is not lighter than glas (a common misconception. It is flexible, and can flex even in frame, if it is large enough. The color is usually clear, but it has a 'plasticy' look to it. It is also a petroleum product, and the cost of acrylic has risen quite a bit.
The main reason to use acrylic is safety. Larger pieces (24 × 30 inches or bigger) can be frankly dangerous when using regular glass. If a piece needs to be shipped, it is probably a good idea to use acrylic, unless the piece is crated vey well, just ot withstand the rigors of transport.
Acrylic is available in an archival form. Like museum glass, it is (no personal experience here) probably quite expesive.
A fair amount of complete ready-made frame packages supply styrene as the glazing. Styrene is a thin plastic, and for the most part is unacceptable as glazing for a piece. It flexes very easily, scratches very easily, and generally looks cheap.