>Please explain to me the difference between a bank and a Ponzi Scheme.

Banks are forced to abide by regulations and laws dictating how fractional-reserve organizations are allowed to operate. In return, peoples' deposits are backed by the government up to $250k (in the US).

Ponzi schemes ignore regulations and laws, and their customers' deposits are not insured or backed by any organization.

The laws that banks are forced to follow may be woefully insufficient, but the government is incentivized to prevent serious collapses because they would be on the hook for a lot of money. Retail customers are also less likely to start a run if they trust that someone will be there ready to hand over their cash when the dust settles.