Grading and drainage are essential for a healthy home, and need to be maintained.  Slab-on-grade house are much more susceptible to moisture problems than houses with raised foundations, unless indirectly from a basement or crawlspace.  However, if water is not provided with a way around a house it will likely find its way inside, and where moisture goes mold and water damage can follow.  Before a house is built, its lot is typically graded to drain water toward a street or other hard surface, but this by itself is not enough. the surfaces surrounding a house should slope away from it at one half inch per foot for a minimum of six (6) feet.  The house should also have hard surfaces and roof gutters that discharge into area drains that convey water to a street or storm drain, and the interior floors should be higher than the exterior grade. 

Rain gutters catch water and carry it through downspouts and on into area drains, or out onto splash-blocks that direct it away from foundations.  Gutters can be made from aluminum, galvanized steel, copper, and plastic.  Regardless of the material, they all need to be cleaned at least once a year, because it doesn't take too long for them to become clogged with leaves, dirt, and debris.  When this happens, the leaves decopose and turn into a soggy mulch that can block downspouts damaging gutters and degrading the wooden fascia-boards to which they're attached.