What are resonant tori?

What is the definition of a resonant/invariant torus (in the phase space of a Hamiltonian system)? Are there non-resonant tori?

Invariant tori in Hamiltonian phase spaces are characterized by two frequencies, one for each dimension: when these frequencies are commensurate ($f_1/f_2$ is rational), curves over these tori close onto themselves and are therefore periodic - these are resonant tori; when the frequencies are not commensurate, the corresponding trajectories are not closed, they are quasi-periodic - and they fill non-resonant tori.