A sensor is usually an analog device. Analog devices are connected at the analog ports of arduino.
Arduino with the use of AnalogRead function can read the analog sensor value from the specified analog pin.
The Arduino board contains a 6 channel 10-bit analog to digital converter. A 10 bit analog to digital converter as we have seen in the "Binary Decimal and Hexadecimal numeral systems" equals to a 210 – 1 = 102410 (0-1023 => 1024 steps).
If the resolution of arduino (1024 steps) does not cover your project you should use an external ADC converter with more that 10 bits.
For example you can connect the LTC2400 ADC which happens to be a 24 bit analog to digital converter.