Plastics are used to make all sorts of things, such as plastic bags plastic bottles and toys.
Most modern plastics are based on organic chemicals which offer a huge range of physical properties to manufacturers — the range of formulations is vast and still growing. There was a time when anything made of plastic was considered to be of inferior quality, but those days are past. You are probably wearing plastic right now — maybe a polyester/cotton mix garment or even spectacles or a watch with plastic components.
The versatility of plastic materials comes from the ability to mold, laminate or shape them, and to tailor them physically and chemically. There is a plastic suitable for almost any application. Plastics do not corrode, though they can degrade in UV (a component of sunlight) and can be affected by solvents — for example, PVC plastic is soluble in acetone.
However, because many plastics are so durable and do not corrode, they create considerable disposal problems. They are not good for the landfill as many will persist for hundreds of years and when incinerated, dangerous gases can be produced. Many supermarkets now give us one-time grocery bags — leave them in a cupboard for a year and all you will have left is dust — they are engineered to degrade. Perversely, some plastics can be cured (hardened) by UV — that just goes to show how varied their formulas are.
We are getting wiser, though, and now many plastics can be chemically, mechanically, or thermally recycled.