Can someone please explain to me what is an estuary habitat?

Now habitats are generally divided into 2 main categories namely, terrestrial and marine habitats. Terrestrial for land and marine for water. Marine habitats are further clified into fresh water(rivers, streams, lakes etc), salt water(seas and oceans), and brackish water habitats. Brackish water habitat is the same as estuary, it is a mixture of fresh and salt water and exists in places where rivers(fresh water) empty into the ocean(salt water). The organisms that live in estuary habitats are those that can neither survive in fresh nor salt water habitats but just require a blend of the two.