A decoder is logic circuit that energizes a particular output line for each combination of input signal.
Fig shown the block diagram logic diagram and function table to a 3 to 8 binary decoder 74LSI138. This particular decoder sends a low signal to one its output lines (Y0-Y7). For example if the input A, B, C, lines at logic 1, similarly if input combination in 110 ( equivalent to 6.) the out put line Y6 goes low.