Glass is made when a mixture of sand, soda and lime is heated to very high temperature into a liquid state. But it doesn't form crystals as it cools, which means it can be molded into almost any desired shape, and it will hold that shape as it cools. Glass is typically very fragile and easy to break. But if it is spun into very tiny fibers, it becomes strong and flexible. That's why glass fibers are often added to materials to make them stronger.
Fiberglass is a composite material. A composite is any material made of more than one component, and therefore has properties of both. Glass fibers are embedded in certain polymers to make them stronger. Fiber-reinforced composites are both strong and light, often stronger than steel. Fiberglass is one such composite. In fiberglass, the glass fibers aren't arranged in any particular direction. By lining them all up in the same direction, scientists can make the composite stronger -- but only in that one direction.
To get stronger composites that can withstand stress from all directions, the fibers can be woven into a kind of fabric "net". Surrounding the fibers with a polymer helps hold the fibers together and also better absorbs force of impact, or stress, so the composite material is tougher than glass. The polymer also keeps the glass fibers from buckling when compressed.
