Last Updated: Sep 19, 2016 Views: 30
David Whitehouse defines "crystal" as:
"A popular term for colorless lead glass which has a high refractive index and consequently is particularly brilliant. Today, the word is often used to describe any fine glass tableware." (From the "Glass Glossary" in http://www.cmog.org)
So, crystal is glass that is considered to be of high quality.
The British Glass Industry website tells you more about lead crystal or lead glass: http://www.britglass.org.uk/AboutGlass/TypesofGlass.html#2
"Commonly known as lead crystal, lead glass is used to make a wide variety of decorative glass objects.
It is made by using lead oxide instead of calcium oxide, and potassium oxide instead of all or most of the sodium oxide. The traditional English full lead crystal contains at least 30% lead oxide (PbO) but any glass containing at least 24% PbO can be described as lead crystal. Glass containing less than 24% PbO, is known simply as crystal glass.
Lead glass has a high refractive index making it sparkle brightly and a relatively soft surface so that it is easy to decorate by grinding, cutting and engraving which highlights the crystal's brilliance making it popular for glasses, decanters and other decorative objects.
Glass with even higher lead oxide contents (typically 65%) may be used as radiation shielding because of the well-known ability of lead to absorb gamma rays and other forms of harmful radiation."
You may wish to read more in the "About Glass" section of their website: http://www.britglass.org.uk/AboutGlass/AboutGlassHome.html
As stated above, lead glass is often used for cutting (carving) or engraving.
I have attached a bibliography of books and other materials that describe glass engraving techniques and another for glass cutting.