What is an Ising model?

"The Ising model is a mathematical model of ferromagnetism in statistical mechanics, invented by the physicist Wilhelm Lenz who gave it as a problem to his student Ernst Ising after whom it is named. The model consists of atoms in a lattice or graph, each of which can be in one of two states, and each of which interacts only with its nearest neighbors. The 1-dimensional Ising model has no phase change and was solved by Ernst Ising (1925). The 2-dimensional square lattice Ising model is much harder and was solved exactly by Onsager (1944). It has a phase change from a disordered state to a state with long range order, and is one of the simplest statistical models with this property. Istrail (2000) showed that non-planar Ising models are computationally intractable, which strongly suggests that in dimensions 3 and above there is no simple exact solution." -wikipedia