Also known as Polyethylene Terephthalate among scientists, PET is a very common plastic. Actually it is the most recycled plastic in the world, due to the fact that it is 100% recyclable. And if you add its other properties like the fact that it is safe, strong, lightweight and flexible, it is easy to understand why it is a very popular material and the base for many products such as water bottles, packaging, clothing or bags!
PET is so common partly because it has been used for so long. It dates back the 40's, when a Dupont's chemist first synthetized it. Then the first PET bottle was commercialized in 1977. So clearly, there is nothing new under the sun, but it is interesting to know the history of a product, and why it has been trusted for so many years.
It was mentioned earlier that PET are 100% recyclable. When recycled, PET is sometimes referred to as rPET, which literally stands for recycled PET.
As an alternative to PET, we sometimes use PP (short for Polypropylene), which is the world's second most produced synthetic plastic.
It is recyclable as well. They do not necessarily have many elements to differentiate them without going too deep into chemistry analysis, but PET bags tend to be shinier.