The document describes the sum-product algorithm, an efficient exact inference algorithm for finding marginals in graphical models. The algorithm works by:
1) Representing the factor graph as a set of factor nodes connected by links to variable nodes.
2) Passing "messages" along the links in both directions between variable and factor nodes. These messages are functions of neighboring variables.
3) Computing the marginal probability of a variable as the product of all incoming messages to that variable node.