A Serewood is a flesh-eating forest. They hide in regular (well, as regular as anything can be in Faerie) forests, and if travelers are not paying attention to the path they may find themselves trapped in one. It is very hard to escape from a Serewood. Even the leaves are deadly--either they are sharp or they bite, it was not made clear. You can't fly out--in the books this was demonstrated by a bird's skeleton, picked clean--they can pixie-lead even people who have been in Faerie all their lives, and presumably you cannot tunnel out because of the roots. Tristan and Charmed only escape because Tristan has the ability to find anything in Faerie--including the safe path through the forest.
Serewoods are characterized by sudden realizations that that path is not actually a path, eerie silence, skeletons picked clean of every shred of flesh, biting leaves, and an indefinable aura of menace.