OK, so we all know what "fresh water" refers to. It's what fills our inland rivers and lakes, the vast majority of its volume created by rainfall. It's what fills your garden ponds and comes out of your taps (filtered and full of chlorine!).
Salt water, is water that contains, er, salt. It has a measurable 'salinity'. It's what fills our oceans, and your marine tank. When water evapourates from the sea surface to form clouds, it leaves its salt content behind, to rain 'freshwater' o'er the land.
The environment created in waterways where freshwater and salt water mingle is commonly referred to as 'brackish'. Coastal estuaries and salt marshes, familiar to many wetland birdwatchers, are brackish. Brackish water will typically contain anything from 5 to 30 grams of salt per litre.
Popular brackish fish kept in aquariums include many types of puffer fish, scats, archerfish and even mollies!
Photo credit: M Chinnery, St. Helens IOW Salt Marshes