A decoder does the reverse of encoder.

It is a combinational circuit that converts binary information from 'n' input line to a maximum of 2n output lines.

The example of decoder circuit would be an AND gate because the output of an AND gate is "High"(1) only when all its inputs are "high". Such output is called as 'active high output'. If instead of AND gate, the NAND gate is connected, the output will be 'low' (0) only when all its inputs are 'high'. Such output is called as 'active low output'.

3:8 decode has 3 input lines and 8 output lines.

It is also called as binary to octal decoder as it takes a 3-bit binary input code and activates 8 outputs corresponding to that code.