An integrating A-D converter measures the input voltage by allowing it to charge a capacitor for a defined period. The integration averages the input signal over the integration time, which if chosen appropriately will average over a complete mains cycle thereby helping to reduce mains frequency interference. The throughput of an integrating converter is not the inverse of the integration time, as throughput also depends on the maximum discharge time. This is useful for long term monitoring since drifts do not accumulate. When the A-D converter is preceded by signal conditioning circuits offset errors need not normally be considered.
Drift occurs because components in the amplifier change over time and with temperature.
You can find these and other technical notes in the Microlink Measurement and Control Systems Catalogue which is available free from Biodata.