Groundwater is everywhere, even in fairly dry climates. In some cases it is simply rain or melted snow soaking in from the surface, in others it comes up in springs. The weight of the surrounding earth, called hydrostatic pressure, forces the water to the surface. If the concrete slab on the bottom of your home is cracked, water can seep through.
Good drainage is important. A perimeter drain system around your house takes the pressure off, absorbing water as it is squeezed out of the ground. However, older houses where the perimeter drain is attached to a city storm sewer that is too small can back up in heavy rain, bringing even more water to the house's foundation.
Check all parts of a wet basement. It's possible it's not groundwater at all, but surface water that is going over the lip of the foundation and down the inside of the wall. Watch for places where yards or beds have built up too high against the sides of the foundation.