I would just like to stress that the reason why + refers to bosons and - to fermions is that the wave function of a system of particles has to be symetric for bosons and antisymetric for fermions upon interchange of two particles. That means that we can write the solution as the product of single particle states which are the solution of schrodinger equation, and since the sum of the solution is also a solution we can form a sum such that the resulting wave function is symetric or antisymeric accordingly. See W. Greiner Introduction to Quantum Mechanics ). This being said one can deduce that if fermion system of particles having an antisymetric wave function were to contain two particles in the same staste psi(x1,x2,...,xk,...,xk,..xn), upon interchange of the two identical particles in the same state we get psi(x1,x2,...,xk,...,xk,..xn)=-psi(x1,x2,...,xk,...,xk,..xn) which implies that the wave funtion vanishes, i.e. no two fermions can exist in the same quantum state. This is just the generalization of Pauli exclusion principle.