Answer:
It really depends on the type of glass. Glass is made from silica, which is the common ingredient in all glasses. From there on everything else varies. One simple thing to remember is that glasses are usually named for their fluxes (with a few exceptions). To get the silica to melt at a lower temperature, a flux is added. Some examples are:
Soda-Lime glass (very commonly used for drinking glasses, windows, etc) has lime and sodium carbonate as flux.
Borosilicate glass contains Borax or Boric acid as a flux
Lead Crystal is a glass using lead as its flux. The lead increases the refractive index, which when cut makes the glass sparkle. Hence it is used a lot in tableware. It was developed expecially for this by Ravenscroft.
Flint glass has flint as its flux
and so-on.
In addition to this glass also contains stabilising agents and clarifying agents.
Coloured glass contains in addition to the above various metal oxides as colouring agents.
for example
cobaltous/cobalt oxide makes blue
selenium or copper or gold oxide can make reds
chromium oxide makes green
tin oxide makes white
silver oxide makes transparent yellows
cadmium can make opaque yellow, orange and red amongst others
some glasses contain combinations of these
The raw mixture of the chemicals glass is made of is called "batch". Often chemicals present in the batch are not present in the melt, as some chemicals are used as catalysts to produce certain reactions at certain temperatures. A chemical change occurs and the catalyst is burned off or changed from one form to another.
hope this helps somewhat