title: In image matching, what is the role of model?
body: <p>I am studying image matching techniques. I am studying an image feature extraction method such as SIFT and matching technique such as Bag of Features. I have been reading few articles about this computer vision field and noticed that they talk about "model" all the time. What is this model and what's the role of a system? They talk about "learning the model." what does this mean??
  Thanks</p>