The order and the nature of the finite-temperature phase transition of QCD with two flavors of dynamical quarks is investigated. An analysis of the critical exponent of the specific heat is performed through finite-size and finite-mass scaling of various susceptibilities. Dual superconductivity of QCD vacuum is investigated using a disorder parameter, namely the v.e.v. of a monopole creation operator. Hybrid R simulations were run at lattice spatial sizes of $12^3$, $16^3$, $20^3$ and $32^3$ and temporal size $N_t=4$, with quark masses in the range $am_q = 0.3 - 0.01$.