Ground Type pokemon are defined by their habit of taking advantage of the environment, whether that means causing an earthquake or grabbing a handy bone to swing at their foe. Most of these pokemon consequently either are very strong or burrow through the ground, although dual watertypes are, understandably, an exception to this rule. Browns and grays dominate this type, though sandy yellows are also prominent. Of course, as with all types, these are only guidelines, not rules.
Ground types instinctively make themselves aware of the lay of the land, feeling out the area's geological stability and the relative hardness of the soil with either vision or burrowing instincts. This makes them instinctively aware of both themselves and their enemies relative positions, as well as allowing them information from which to plan an assault. Because their abilities, in general, are based on subverting the stability of the environment, they tend to be very good aggressively; the heavy rock and steel types fall to their tunnels easily, and the glands of both fire and poison types react poorly to a thrown clod of dirt. Electric types in particular suffer against ground types, as they not only will cover the electrics in heavily debilitating mud but will also ground themselves, making electric attacks worthless. Their strength and/or burrowing prowess also makes them resistant to the attacks of poison and rock pokemon, since they will shrug off or dodge it to a mild degree.
On the flip side, their reliance on the environment as an assault mechanism means the smaller bug types and the more environmentally aware grass types do not suffer as much from their assaults, and flying types can evade their attacks entirely. Environment-altering attacks can also cause ground types much suffering; water and ice types wash away/freeze the soil, causing ground types to misjudge their assaults, and grass types can break apart the very ground the pokemon rely on with their rootlets.
Ground types are generally built either to burrow through the ground or to use their strength to their fullest.
Because a large number of ground types live underground, roots and burrowing insects seem to be a food of choice. However, not all ground types eat these and some should in fact be fed in large quantities.
Widely varied. Ground types tend to raise their young to adulthood, though.