You can control the temperature either by typing a (positive) real number into the temperature field or by adjusting the thermometer by mouse. To set the critical temperature TCrit=2/ln(1+sqrt2) of the 2 dimensional Ising model (which is connected with a phase transition on the infinite lattice) click on the corresponding symbol. Every little box of the spin field represents one of the two possible states Si=-1, 1 (white/blue).
The Hamiltonian is calculated by the formula H=Sum<i,j>SiSj where <i,j> symbolizes all pairs of next neighbours on the lattice. The magnetization is simply the mean of all spins.