Q. What makes crystal glass different from general glass?

David Whitehouse, Former Museum director, 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 Disctionary") 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: 

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. The lead is locked into the chemical structure of the glass so there is no risk to human health. 

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. 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. If you wish to obtain copies of any of these items, please contact your local library. The Rakow Research Library is a member of the Online Computer Library Center (OCLC); your library can contact us through OCLC or they may mail an Interlibrary Loan form to us. You may request up to five items at a time. We send photocopies of articles and microfiche copies of our books (if they have been microfilmed) through Interlibrary Loan for four weeks use.