Digital data can only have distinct values.

Examples: 

• a light switch - it can only be 'on' or 'off'
• the number of cars passing a point in a road.
• the number of people attending a football match

Analogue data can vary continuously within a range of values.

Examples:

• the temperature in a greenhouse - it can be any numerical value.
• a person's height
• sound frequencies

A computer is a digital device and so it can only process digital data. So if an analogue signal is sent to a computer, it would have to be converted to a digital signal first. What does this conversion? ...yes! it is done by an analogue-digital converter.

Most often found in control systems
Examples :  

[1] A computer could be monitoring the temperature of a manufacturing process. The temperature arrives as analogue data and must be converted to digital before the computer can process it.

[2] A speech recognition system would use microphones to input the sound, but the analogue sound data would need to be converted to digital data before the computer could process it.