To add to Sullivan's great response, there is a variety of internal standards that can be used depending on the type of analytical method.

Internal standards can also reduce matrix effects when they are heavy-isotope labeled standards of the analyte of interest and mass spectrometric detection is applied.

The main difference between internal standard and standard addition is that the first is better for larger number of samples and most often still requires a calibration curve, whereas the latter does not necessarily require a calibration curve and is better for smaller number of samples with large matrix effects.

Thanks Stefanie Maedler & Sullivan Lee