Defensively, the Ground-type is somewhat risky. It is weak to Water, Grass and Ice, which are key attacking types. It resists Poison, a rare offensive type, and Rock. It is, however, immune to the Electric-type, posing a great advantage due to the minority of Pokémon resisting Electric-type moves that are not Electric-types themselves, and the lack of a ¼ resistance to Electric even by Generation IV.
Offensively, the Ground-type is one of the three most important attacking types, together with Ice and Fighting. It is the only type super-effective against Electric, and is super-effective against five types in total; only the Fighting-type is as powerful as this. It is also super-effective against the best defensive type, Steel. Grass- and Bug-types do resist Ground, but many of them lose their Ground resistance due to being part Poison. This, combined with the fact that the highly damaging, very accurate and rather popular (especially among physical attackers) move Earthquake has been available as TM26 in every generation so far, leads to Ground being very potent offensively.
The greatest disadvantage of the Ground-type is that it does not affect the very common Flying-type at all, or Pokémon with Levitate.
The Ground-type combines very well with the Ice-type, as Ice covers Grass and Flying, while Ground covers Rock, Steel, and Fire. Ground and Rock also work very well together; Rock covers Ice and Flying while Ground covers Steel; as of Generation IV, only Claydol, Flygon, Bronzong with Levitate, Breloom and Torterra resist both Rock and Ground. Most Ground-type Pokémon are able to learn Rock-type moves, possibly for that very purpose.
When used in contests, Ground-type moves typically become Tough moves. No Ground-type moves are Beauty moves, while they can be the other three types.
In total, there are 48 Pokémon with the Ground-type.