 Ferns are a very ancient family of plants: early fern fossils predate the beginning of the Mesozoic era, 360 million years ago. They are older than land animals and far older than the dinosaurs. They were thriving on Earth for two hundred million years before the flowering plants evolved.
As we know them now, most ferns are leafy plants that grow in moist areas under forest canopy. They are "vascular plants" with well-developed internal vein structures that promote the flow of water and nutrients. Unlike the other vascular plants, the flowering plants and conifers, where the adult plant grows immediately from the seed, ferns reproduce from spores and an intermediate plant stage called a gametophyte.
Ferns are (relatively) delicate plants that only grow in areas where there are suitably moist conditions. They favour sheltered areas under the forest canopy, along creeks and streams and other sources of permanent moisture. They cannot grow readily in hot dry areas like flowering plants and conifers.
Ferns can only reproduce where there is sufficient moisture: the reproduction process itself requires moisture.
Like all plants, ferns have evolved to suit their environment. Some can tolerate extreme drought and heat, others only live in the deepest rainforest. You can't grow a cactus in the sub-Antarctic, though grasses will survive there. And likewise, you can't expect a tree fern to grow in a desert, though some varieties of rock fern do.
Ferns are very successful niche plants: they are well adapted to particular environmental niches - soil moisture, humidity, light, etc. They seldom grow outside these niches, some of which are very specific.
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