The broker-to-carrier spot rate is the rate paid by brokers to carriers and owner operators. These are the pay to truck rates paid by transportation brokers. These rates are not governed by a contract or long term agreement so they are spot rates negotiated on a day to day basis. Because they are rates to the carrier, spot market rates exclude the margin brokers earn by arranging transportation and providing other services.
The shipper-to-carrier contract rate is the rate shippers under contracts pay carriers that have been negotiated up to a year beforehand. Typically mid-sized and large carriers as well as brokers bid for business directly with shippers. When these rates are between a shipper and a carrier they include the sales and customer relations expenses of the carrier. When they are with a broker they include the brokers margins.
Shipper to carrier contract rates can be thought of as the "retail" rate for freight that is planned and regular freight.
Broker to carrier spot rates can be thought as the "wholesale" rate for freight that is irregular, seasonal, or unexpected.