An amphibious aircraft can take off and land both on conventional runways and water.
A true seaplane can only take off and land on water.
Amphibious aircraft are typically flying boats and floatplanes with retractable wheels.
There are amphibious flying boats and amphibious floatplanes, as well as some hybrid designs, e.g., floatplanes with retractable floats. Modern production seaplanes are typically light aircraft, amphibious and of a floatplane design.