Answers:
a 'wholesaler' sells things in bulk (by the dozens to a hundreds or even by boxes or crates) and usually buy them at a cheaper price because of the large quantity purchase. the 'retailer' sells items in retail or in small quantities of course you pay a higher price for an item sold based on per unit/item purchase. sometimes you will see 'wholesaler-retailer'- it's just a combination of both. the customer can choose which purchase option suits him best.
on the other hand, the distributor serves as the middleperson from factory to wholesaler, from wholesaler to retailer and sometimes even from retailer to the buyer that relays and transports particular products to the clients they cater to. nowadays, businesses try as much as possible not to make much use of middlepersons to reduce costs in their transactions.