The foundation supplies the base for your entire home and transmits the load of the structure to the supporting soil. It consists of a footing that supports a load-bearing wall (basement wall) and a concrete slab.
Because the foundation and basement are surrounded by soil, water can leak or seep into the home through cracks or holes, or through mortar between concrete blocks.
Besides problems with moisture, you might find that your foundation/basement system settles or allows radon gas to penetrate.
Here is what footing and drain tiles look like: