Where a job lives, how it is saved, and the storage locations it can live on.
A project is the container for a job: the sheets, every measurement and markup on them, the item lists, the layers and the layout you left it in. It lives on a storage location you set up once.
You can also open a PDF straight from disk without making a project of it. That is a quick look: the canvas comes up clean, and work done there is deliberately not kept. When a quick look turns into a job, File > Save as project promotes it without starting over.