pyrex is a name giving to a formula of glass from corning. Most know pyrex as the glass cooking ware. That however is a different formulation then the one we use . Other common terms for the glass I use are borosilicate or lab grade pyrex. Basically glass can be made up of many components like some has lead, some has soda lime, some has boric acid(pyrex) mixed with silica.
Lab grade pyrex is a very stable formulation of glass that resists acid and heat extremely well. It also has excellent optical clarity. Its often called hard glass as its very stiff to work with and requires an intense amount of heat to bring it to the point of softening where it can be manipulated. If you've seen anyone blowing glass in your life most likely you were witnessing "soft" glass not pyrex.
Its also an extremely expensive glass costing many times that of what most "soft" glass. Hope that helps.