An DAC convert digital signal to analog signal i.e Digital to Analog Converter. An ADC convert analog signal to digital signal i.e Analog to Digital Converter.

An analog-to-digital converter is a device which converts an infinite resolution analog signal to a finite resolution digital signal.

Digital to analog converter.
An analog to digital converter and digital to analog converter are present in a computer. they serve this purpose. They are present in the form of ICs inside.

where an analog signal is to be converted into digital signal,because in some applications we need digital signal where we have analog signal that is we use an A/D converter

Well you can use Digital-Analog Converter to do the same operation.