is a specific kind of: fern.

is a broader term that covers: clover fern, pepperwort | pillwort, Pilularia globulifera | regnellidium, Regnellidium diphyllum | floating-moss, Salvinia auriculata, Salvinia rotundifolia | Azolla caroliniana, Carolina pond fern, floating fern, mosquito fern | Ceratopteris pteridioides, floating fern, water sprite | Ceratopteris thalictroides, floating fern.