You have raised an excellent question. Both can sell new or used clothing, but they do have their differences.
Consignment utilizes a 3rd party vendor to sell product such as clothing, fashion accessories, or shoes. OK, maybe you are not sure the meaning of a 3rd party vendor. Just in case you are not certain what we mean by a "3rd party vendor". A third-party source means a supplier (or service provider) who is not directly controlled by either the seller (first party) nor the customer / buyer (second party) in a business transaction. For example, if you want to sell a pair of collectible sneakers on consignment, you could drop off your collectible sneakers at a consignment shop (3rd party) and they will sell to a customer on your behalf. Obviously, they will want a fee for the transaction.
Although consignment retailers can sell new clothing, shoes, jewelry, etc., they often sell used second hand clothing.
A thrift store is a retailer that typically sells donated product to raise money for charity. They can also be a for profit business. Although they can sell new clothing, shoes, jewelry, etc., they often sell used second hand clothing. Similar to a consignment shop, they can sell new clothing, footwear, accessories etc., but most often sell used second hand clothing.
A thrift store takes owner ship of product to sell. A consignment shop sells product on behalf of someone else.
You may want to learn about second hand clothing if you enjoy shopping at either a thrift store or a consignment shop.