Bill of Lading (B/L)
A bill of lading is the carrier's document that acts as receipt for the cargo, evidence of the contract of carriage, and — when negotiable — document of title.
Short answer: A bill of lading is the transport document a carrier issues for a shipment. It does three jobs at once: it receipts the cargo, it evidences the contract of carriage, and if it is negotiable it is the document of title that controls who may collect the goods.
A bill of lading (B/L) is issued by the carrier or its agent once cargo is received. It is the single most consequential document in an ocean shipment, because it can determine who is legally entitled to take delivery.
Master versus house
A master bill (MBL) is issued by the ocean carrier to the freight forwarder. A house bill (HBL) is issued by the forwarder to the actual shipper. One container can therefore sit under two bills at once, which is why reconciling them is a routine part of receiving.
Negotiable or not
An original (negotiable) bill must be surrendered to release the cargo — it is title. A sea waybill or express release is non-negotiable: the named consignee takes delivery on identification alone, with no paper to courier.