Shipping containers, whilst appearing to just be a hunk of steel on the outside, are actually one of the world's most versatile storage solutions. They're used for freight to cross the seas day in and day out, but many people don't realize that you can actually buy them (very easily) from the internet for your own personal use.
Shipping containers are are made initially to be incredibly secure- i.e., let in no water, wind, residue, damp, etc. They are probably the sturdiest non-built storage option out there. You're probably familiar with shipping containers given that they're highly visible- they make up a large majority of all overseas trade, and are traditionally stacked in shipping ports.
In this context, shipping containers give away their primary function in their name- they ship things! They're the favorite choice for businesses that need to transport cargo internationally for trade. That's largely down to their sturdiness and reliability, as well as the fact it's much cheaper to ship cargo this way than to send it via air travel.
In the cargo shipping industry, they're often referred to by a multitude of different names. 'Conex box', 'ISO box', 'ship container'- the list goes on. Each of these terms is completely fine to use as they are all interchangeable.
The only difference between shipping containers that are used for cargo shipping and those that are used as storage solutions on-land is that cargo containers must adhere to a set of standards outlined by the International Organization for Standardization regarding their size and stability. They must receive a certificate that rules them as cargo worthy in order to legally be used as cargo shipping containers.
Many of the shipping containers you see being used as storage on land are often second-hand containers that were formerly used for shipping cargo, but are no longer suitable for traveling across the seas. In this sense, their condition is usually slightly worse than those containers in shipping jobs, but they're still in a good enough condition to be used for storage or other on-land purposes.