Block, File, Object Data: What Does This Really Mean?

All data is stored in blocks on disk. Users access data as files. I get this.

So then what's the difference between block-level and file-level access? Even disk is formatted with NTFS so users can interact with data stored on DAS or SAN. So then what does block-level access really mean - and what exactly dictates whether data will be block, file, or object?

I know NAS has a special OS that translates file to block for writing, and block to file for retrieving. I don't understand when people say NAS is great for "file-level access." So, then what type of access is an end user getting from a SAN if it's not file-level?

And does the software on Centera and Atmos transform data into objects? When do they become objects and is this data ever files?

I guess I'm hoping for an explanation that doesn't just tag NAS with file, disk/SAN with block, and CAS with object. I'd like to know if theres any difference for the client, and where the real benefit is to having 3 different types of data access.