My best guess:
The dull answer is, dryads are fey so they mostly spontaneously arise in oaks in very old forests. They are exclusively female so they do not have any mating habits per se. They are the souls of the trees.
Dryads occassionally take fancies to other creatures and being fey can pretty much 'enjoy the company' of anything. As fey are magical creatures, there is a reasonable chance the dryad and her swain could interbreed. The child would either be a dryad (if the dad is another kind of fey such as a satyr) who would be born at the same time as an oak sapling germinates from an acorn off the parent tree. The oak/dryad soul-link starts at conception.
If daddy isn't fey (and he could be anything give the general free spirited nature of dryads) then the child is half-fey. He or she would not be bound to a tree but would likely feel an affinity to nature. Elves are the most likely candidates for relationships with dryads as they share similar outlooks and livespans but other crosses are entirely possible.
The Dungeon and Dragons open source supplement 'Bastards and Bloodlines' describes a half-dryad as a 'Spring Child' and goes on to state that dryads are affectionate mothers but find it difficult to relate to their mixed heritage children. There is little to a dryad's life other than her tie to her oak and the woods beyond so the 'outside world' and its concerns are foriegn to her. That makes it difficult to half-dryads to share their experiences with their mothers as dryads cannot stray far from their trees.
Nihil est ab omni partum beatum.
(Nothing is an unmixed blessing)