Many of the techniques of NLP were created through a modelling process. For example, the psychotherapy aspects of NLP were created by modelling people such as Milton ERICKSON and Virginia SATIR. In fact, some actually say that NLP is a modelling process. In all cases, modelling is a core NLP technique.
Modelling isn't really something new, as people have used modelling much before NLP in various forms. For example, people try to observe how top achievers succeed in their dedicated fields and try to formalise what they observe to make a model which could be used by themselves or others. This kind of modelling is somewhat 'basic' in that the modellers focused on the observable aspects of the people being modelled.
In contrast, NLP modelling adopts a systemic approach (like most of NLP) by taking both the observable and non observable processes into account for the modelling. NLP modellers use NLP techniques in order to model things such as internal representations, states, meta programs and physiology. This gives them information on how the person is operating in addition to knowing what operation he is doing.
Thus, NLP modelling involves paying attention to what the model is doing and also what are his internal processes. The first aspect is quite natural and is common to most modelling methods while the second is more specific to NLP. NLP provides many guides in order to uncover what's happening internally. For example, it's important to note the model's usage of modalities and sub modalities to help figure out how he is using his internal representations. Identification of used meta programs is also another valuable information, knowing if the model is pushed from threats or pulled to opportunities for instance. Understanding what the model's state is, how he's filtering his perceptions, his strategies, his values, beliefs and attitudes, ...
Using NLP modelling correctly gives good results fast allowing people to improve or learn new skills in a most efficient fashion. The KM KCard gives some more information relevant to this topic.