A chemical that is more toxic to some plant species than to others (may be a function of dosage or mode of application).
One that kills specific undesirable plants, sparing other desirable plants; this is done through different types of toxic action or by the manner in which the material is used (its formulation, dosage, timing, placement, etc.).
A chemical that kills plants in certain classes but not others. The contrasting term is "non-selective" herbicide, a chemical that kills all or nearly all plants.