Ground-type Pokémon have powers and abilities related to control of ground and earth. Ground-type Pokémon are afraid of water, like Rock-type Pokémon, unless they are Water type. Many Ground Pokémon are also partially Rock type.
These Pokémon are normally found in caves or rocky terrains, with the exceptions of some dual typed Pokémon. One bad thing about them is that they can't normally come off the ground, such as Diglett and Dugtrio, even though they are pretty good combatants when they are below the Earth's surface. They have powerful moves such as Earthquake and Fissure.
They're strong against the Fire type because ground, mud and sand are usually used to put off a fire, that's because the ground suffocates the fire, letting it without any oxygen.
They're strong against the Poison type because if you drop any venom into the ground, this ends by disappearing.
They're strong against the Rock type because a sandstorm can erode rocks, also because an earthquake can break rocks.
They're strong against the Steel type because a sandstorm impedes machines like cars, because the oil lubricates their gears and with sand, dirt, or mud, the oil doesn't work. Also, earthquakes destroy metallic structures.
They're weak against the Grass type because plants absorb minerals from it and open through the ground using their roots.
They're weak against the Ice and Water types in reference of erosion. Also because earth becomes weak when it's wet or frozen.
They're immune against the Electric type because in real life, the ground is a natural sink of electrons, and electrical appliances don't work when filled with dirt. Also, earthquakes destroy power lines.
The Bug type resists them because many bugs like worms live inside the ground and many of them eat it. And burying a bug in sand, dirt, or mud will result in the bug digging its way out.
The Flying type is immune to them because something that is flying doesn't touch ground, therefore, they don't get affected by earthquakes and Earth shifts.