Maxim describes the theory behind Delta-Sigma ADCs:

This in-depth article covers the theory behind a Delta-Sigma analog-to-digital converter (ADC). It specifically focuses on the difficult to understand key digital concepts of over-sampling, noise shaping, and decimation filtering. A description of new converter, the MAX1402, and several applications for Delta-Sigma converters are included.

The 16bit ADC on some MSP430s use this approach. Here’s a list of different ADC techniques.