HART = Highway Addressable Remote Transducer-. is a communication protocol.
SMART = Single Modular Auto-ranging Remote Transducer.
SMART instruments can be HART,FF or Profibus.
SMART transmitters compared to analog transmitters have microprocessor as their integral part , which helps for self diagnostic abilities , non-linear compensations , re-ranging without performing calibrations ,and ability to communicate digitally over the network.
HART standard helps instruments to digitally communicate with one another over the same two wires used to convey a 4-20 ma analog instrument signal.