In this talk, I will present a general approach to study spin systems with two symmetric absorbing states. I will first give a short introduction on interacting particle systems, showing some of the most common examples that probability theorists have investigated, and then I will focus on a subset of them, spin models. Starting from the microscopic dynamics on a square lattice, it is possible to derive a Langevin equation for the time evolution of the magnetization field, that explains coarsening properties of a wide range of nonlinear voter models. It turns out that the macroscopic behavior only depends on the first derivatives of the spin-flip probabilities. To illustrate these results, I apply this approach to study models with intermediate states. Finally, I show how a mean-field approximation reveals the three types of transitions commonly observed in these systems -generalized voter, Ising and directed percolation-.