Pyrex was the brand for Corning's borosilicate glass products, which is an almost all-around superior substitute for regular soda-lime glass. Better acid resistance, lower thermal expansion, higher softening/melting temperature, etc.

Some marketing geniuses somewhere concluded that consumers wouldn't notice, and created the line of products that carry the confusing "pyrex" branding but aren't borosilicate based but just thicker or something.

And now they have to have this page to explain the differences.