Discrete semiconductors are individually packaged devices or arrays of individual devices (transistors, diodes, SCRs, etc.) that operate at signal levels or are used in power applications.
Diodes are two-terminal semiconductors that allow current to flow through in only one direction.
Thyristors are a family of semiconductor devices that exhibit bi-stable current-voltage characteristics and can be switched between a high-impedance, low-current off state and a low-impedance, high-current on state.
Transistors are three-terminal semiconductors which use an input signal controls to output current. They are normally used for amplification and switching applications.