Foundations are built in holes in the ground, and most ground is full of water. Water will take the easiest path to move through the soil and, unfortunately, your foundation is one of those paths. Concrete absorbs water, and the soil that is filled around your foundation is looser and has more air in it than the rest of your property. There are compounds that contractors can apply to the exterior foundation walls to prevent moisture from seeping in, but the other thing that needs to be done properly and professionally is having a drainage system installed.
Foundation Drainage
The key element to foundation drainage is what is called the "drain tile." This is a pipe that has perforations or holes in it that allow water to enter it. It encircles your foundation at the footer (the slab of concrete at the bottom of the foundation) and drains water to another location.
The system generally looks like this: a trench around the outside of or on top of the footer, a layer of gravel, the drain pipe, and then more gravel on top of that. Some contractors believe that there should also be a layer of roofing felt, tar paper, or some other filter to keep silt from going through the gravel and clogging the drain tile.
If your house is on a slope (which many are), then the drain tile will run through the ground and come out into the open somewhere on your property. The water will just drain from around your house down to the bottom of the hill. If your house is on level ground, you may have a sump pit installed in your basement, to which the water is drained and then pumped away from the house.If your house is very old, you may have a wet basement, and it may be due to a bad drainage system or a complete lack of one. They can be installed after a house has been built, but the cost and time involved are going to be sizable. Keeping this in mind, do your research and make sure your contractor does a good job of installing your drainage system!