One definition of a hill is high ground up to 307 m high. Above that height it is a mountain. Mountains are solid rock; hills can be solid rock of piles of debris built up by glaciers or the wind.
Hills that are solid rock are either very old, having been worn down from mountains over millions of years, or they are made from soft sediments that were low hills.
In moist climates hills are often rounded by weathering and by water running over the land. As solid rock is weathered, the hill is covered in a layer of debris called regolith. This material either creeps slowly downhill or slumps suddenly in landslides.
Hills often have a shallow S-shaped slope. Geologists call this kind of slope "convexo-concave" because there is a short rounded convex section at the top, and s long dish-shaped concave slope lower down.
Hill slopes become gentler as they are worn away, because the top is worn away faster. This is colled decline. Retreat is where hill slopes stay equally steep, but are simply worn back.
Replacement is where hill slopes wear back, with gentler sections becoming longer and steeper sections shorter.