A foundation drain is a 4″ diameter continuous length of perforated piping that runs the circumference of the basement walls. It is a very important part of the waterproofing system that should surround your house. It lays on a bed of gravel collecting the ground water that tries to force its way into your basement and will reduce the chances that you will get a wet basement.
So you might be asking yourself what the parts of a waterproofing system are and why are they so important. A water proofing system consist of a sealant which is coated over the outside of the basement walls. Obviously it is very important to have the sealant spread on the outside of the basement wall before the excavator comes to backfill. The sealant differs depending on the type of soil that is present.
If your soil is well-drained then the need for a super protective layer is not as important because the water will have as much chance draining back into the soil as it would to enter your house. The coating in well drained soil will usually consist of some portland cement mix that has a fiber reinforcement and then possibly a coating of fiber-reinforced asphalt mastic or tar.
If you soil is wet and poorly drained then the coating need to be more protective. The coatings will consists of a double layer of the portland cement mix or two coats of bituminous waterproofing and then a heavy toweled-on coat of cold, fiber-reinforced asphalt mastic.
Regardless of your soil type the bottom line is that the ground water that may reach your sealed basement wall will travel, because of gravity, down to where the foundation drain will capture the water and drain it away to a location away and below the level of your house.