Freshwater environments are wetlands, lakes, ponds, rivers, and streams where the water has a very low salt content. Animals adapted to this environment can't survive in high salt content places like the oceans, but freshwater is vital to all life on Earth.
Ponds and lakes can range from a few meters to a few miles wide. Some last only a few months, while other can last for hundreds of years. They tend to be isolated from other water sources, so there is little diversity of plants and animals, but that isn't to say there is little life in them. Often they have a few different species of plants and animals that occur in very large numbers.
Rivers and streams are flowing, or moving water sources. They move in one direction, (usually downhill. XD) and can be found everywhere. They have a source which can be anything from snow or glacier melt, springs, or even a lake, and flow to their end or mouth which can open into another river or stream, or even to the ocean. The water also changes as it flows. From being cold and clear at the source, to warm and murky at the mouth. As the water changes the plants and animals change with it.
Wetlands are not exclusively freshwater as some have very high salt contents, such as salt marshes. Still several are including bogs and swamps. Wetlands are areas of standing water that support a large number of different plants and animals.