EDIT: As this is a deep topic, there are also books if that's more your style: http://www.amazon.com/dp/0123725380/?tag=stackoverfl08-20
or maybe you like Wikipedia (Ol' Trusty): http://en.wikipedia.org/wiki/Feature_detection_(computer_vis...
There is a whole chapter on shape contexts in it, which I use with shapecatcher, too.
EDIT: As this is a deep topic, there are also books if that's more your style: http://www.amazon.com/dp/0123725380/?tag=stackoverfl08-20
or maybe you like Wikipedia (Ol' Trusty): http://en.wikipedia.org/wiki/Feature_detection_(computer_vis...