What is difference between BUC and LNB?

BUC (Block Up converter) is used in the satellite communications uplink， 

LNB (Low Noise Block Converter) is used in the satellite communications downlink.

BUC (Block Up converter) It converts the radio signals from a lower frequency to higher to transmit to the satellite. BUC used in remote locations are often 2W or 4W in the Ku band and Ka band, 5W in the C band.

LNB (Low Noise Block Converter) It receives the radio signals from the satellite and converts them to a lower level  (IF band or L band) sent to the cable modem.

An example of a system utilizing both a BUC and a LNB is a VSAT system, used for bidirectional Internet access via satellite.

More: https://www.youtube.com/watch?v=1jn_ybSBVWE