What is the difference between the Lookup and Setup sections?

Lookup and Setup are sometimes used interchangeably, as you can do some of the same things in both sections (for example adding a customer, viewing the bank account ledger). 

However, the Setup section has more in it, compared to the Lookup section. You are also able to delete things (e.g. a supplier) in the Setup section, but you cannot delete things in the Lookup section.