Your scuba Cylinder is a high pressure vessel made specifically for diving. We use two different types in diving with different working pressures.
Steel cylinders are heavier and smaller but can go to higher pressures 232 or 300bar. Aluminium cylinders are larger and more buoyant but most have a maximum working pressure of 207bar but will not rust.
If you want more advice on choosing the best cylinder for you read our advice pages all about Choosing a Dive Cylinder