The sender is the party that concludes the contract of carriage and hands over the goods for transport. It is also called the shipper or consignor; the receiving party is the consignee.
In the CMR note: sender, consignee and carrier are entered by name.
Not always the same: the sender, the loader (who physically loads) and the ordering party can be different companies – who pays for transport often depends on the Incoterms.
For the carrier: the ordering party is the one to negotiate the freight rate with.