Decoder: In Digital Electronics, discrete quantities of information are represented by binary n codes. A binary code of n bits is capable of representing up to 2  distinct elements of coded information. A decoder is a combinational circuit that converts binary information from n input n lines to a maximum of 2 unique output lines.  

(2 to 4) Decoder: The (2 to 4) decoder consists of two inputs B1and B0, and four outputs D0 D1 D2 D3.  (4 to 2) decoder decodes the information from 2 inputs into a 4-bit code.