Nymphs were spirits of fields, woods, streams, of nature in general, personifying its fecundity and gracefulness. They were considered secondary deities, nevertheless, they were respected, sometimes feared and prayed to.
See: Jean-Baptiste-Camille Corot. A Morning. Dance of the Nymphs. A Nymph Playing with Cupid.
Lucas Cranach the Elder The Water Nymph, The Nymph of the Spring.
Jacob Jordaens. Cupid and Sleeping Nymphs.
Edouard Manet. Surprised Nymph.
Titian Shepherd and Nymph.
Naiads were water Nymphs. They were mortal, but were granted very long life. They inhabited springs and small rivers. They could give the waters healing properties, which could help people; and vice versa, if they were displeased, they could make a person, who bathes in their waters, sick.
See: Henryk Siemiradzki. Naiads.