Glass is a hard, brittle and transparent solid that is non-crystalline because its atoms are randomly arranged. It is composed mainly of silicates like silicon dioxide and an alkali. Glass does not have a definite melting point but softens gradually when heated. The main types of glass are soda lime glass, soda-lead glass, borosilicate glass, and fused silica glass, which differ in their composition and properties. Glass is widely used for windows, bottles, scientific equipment, and other applications due to its optical properties and durability.