I attached a good book of S D convertters.
Delta modulation is a Data Communication Technique for transmit Analog Data whit discrete voltage levels (digital).
It samples the signal at a high rate (more than twice its bandwidth) and calculate the diference between the current sample and the last sample. If the difference is positive, you transmit a positive pulse, and if the difference is negative you transmit a negative one. The interesting thing is that you just send one "bit" per sample.
This technique is a special case of Differential PCM (that encodes those sample differences with mor than one bit).
This method has limitations (you can transmit analog data with a given slew rate)
Sigma delta is a modification of that way of transmitting data. It kills Delta Modulation limitations. Also, it has better characteristics facing noise issues.
This system can be seen as a diferentiator a Delta modulator and a integrtor in serie. That change introduces those benefits. This technique is a good way of samplig analog data for implement ADCs (with a post downsamplig)