Storage refers to the media and methods used to keep information available for later use. Some things will be needed right away while other won't be needed for extended periods of time. So different methods are appropriate for different uses.
Earlier when learning about processing, we saw all the kinds of things that are stored in Main Memory.
Main memory keeps track of what is currently being processed. It's volatile, meaning that turning the power off erases all of the data.
For Main Memory, computers use RAM, or Random Access Memory. These memory chips are the fastest, but most expensive, type of storage.
It is nonvolatile, meaning that turning the power off does not erase it.
Auxiliary Storage is used for:
So, Auxiliary Storage is where you put last year's tax info, addresses for old customers, programs you may or may not ever use, data you entered yesterday - everything that is not being used right now.