The most obvious difference between a fish and an amphibian is that (most) fish can only survive when submerged in water. Amphibians, on the other hand, can survive both in and out of the water. Amphibians have legs and feet that help them move about on land. Fish only have fins, which work well in the water but are useless on land.
Fish breathe through gills. These organs need to remain wet in order to pull oxygen out of their environment. Gills dry out quickly out of the water, causing the fish to suffocate. Amphibians can breathe just fine in or out of the water. They have the ability to absorb oxygen through their skin whether on land, although they need to remain wet for this, or in the water. Unlike most fish, they have lungs that allow them to breathe even more efficiently from the air.
Most fish are covered in scales. This provides them with a degree of protection and also makes them more hydrodynamic. Some fish, like sharks and catfish, lack scales but have a tough layer of skin. Amphibians do not have scales and their skin is semipermeable. This allows them to drink and breathe directly through their skin. However, it offers them very little protection.
Different fish species can be found widely in both fresh and saltwater. There are some, such as salmon, that can tolerate both environments. Nearly all amphibians are found exclusively in freshwater. Some can tolerate brief trips into brackish water, but their semipermeable skin means that they would desiccate if exposed to saltwater for too long.