Generally we use many sensor for most of our circuits. Sensors are usually provides Analog signals it converts physical quantity to analog values there we need ADC nothing but Analog to Digital Conversion.
There exist several methods to convert analog data to Digital data but it is replaced by IC in earlier days we use ADC0804. Which is single channel input pin and 8 channel output pin. only one drawback is we have to program it for conversion here it uses Successive approximation
Nowadays we use ADC0808/0809 I has 8 input channel and 8 output channel the major advantage is the SOC, EOC, RD are in build and automatically done by timer IC. Here we can interface 8 sensor we can select the sensor among them.