There are several basic types of glass for picture framing.
Standard clear glass. Self explanatory
Non Reflective Glass. Also known as diffused glass. Acid etched on one or both sides. This glass, although helping reduce reflections, can alter colour and blur detail.
UV filtering Glass. Also known as Conservation glass. Can block up to almost 100% of UV light to help prevent fading* Looks no different to standard glass.
Optically Coated Glass. Also known as Anti-Reflective glass. Head on, in normal lighting conditions, this glass is literally invisible, see photo, The reflection on the lacquered frame has not continued on to the glass,