# Boltzmann machines: learning algorithm

I'm trying to study Boltzmann machines, so I don't undestand this recurrent formulation for the training stage of the weights $w$:

$\Delta w_{ij} = E_{data} (v_i h_j ) − E_{model} (v_i h_j )$

all references tell that $E_{data}$ is the expectation observed in the training set while $E_{model}$ is "that same expectation under the distribution defined by the model"; I don't understand what is this "expectation of the model" and why is intractable; is there a clear reference to understand this concept that is still unclared to me?