Ellen Talbot lays out some of the basics of image recognition:
Aggregate channel features (ACF) is a variation of channel features, which extracts features directly as pixel values in extended channels without computing rectangular sums at various locations and scales.
Common channels include the colour channels, such as grey-scale and RBG, but many other channels can be encoded, depending on the difficulty of your problem (e.g. gradient magnitude and gradient histograms).
ACF has advantages, such as a richer representation, accelerated detection speed and more accurate localisation of objects in the images when used in conjunction with a boosting method.
Click through for more, including a few resources around the Viola-Jones algorithm.