Amphibians have a wide variety of features that allow them to be both aquatic and terrestrial.
Amphibians are widely terrestrial in nature. In fact the word amphibian comes from the Greek words amphi (both or double) and bio (life or lives). Amphibians exclusively live in the water during their early stages of development. Their external set of gills allow them to focus oxygen from the water into their bodies.
Later on in their lives their tails are lost due to a need for fundamental chemicals elsewhere in preparation for radical amphibious morphological change. Also the hind legs develop beneath the gill sac in preparation for land. This period is known as metamorphosis ( literally changing form). The internal gills that once worked to their advantage in the water turn into external gills that can utilize oxygen on land. Of course the stages of metamorphosis vary slightly in different of amphibians.
Amphibians depending on the species usually have strong legs and a wide gait (stance) to support their body on land. In addition many amphibians develop moist skin so that they can breath through their skin and lungs interchangeably (oxygen transfer requires a moist membrane). Also amphibians develop stereoscopic vision (the slight variance in the images of both eyes) so that they can see in three-dimensional depth. This adaption (also seen in humans) allows for more efficient navigation and amphibian feeding tactics on land.
Here is a simple visual that illustrates the life cycle of a frog one of the most common and varied amphibians in the world.