Some eco-conscious people have set up such systems to use their "gray" used water for irrigation. You can stop this by rerouting the water draining from the washing machine. If you can't do this yourself, a plumber should be able to handle it pretty cheaply and easily.
I have also seen where "pressure relief" lines are run from the hot water heater to out side...sometime these can be confused with other lines.
This pipe is an overflow pipe that protects the inside of the house from water damage should something break such as a water hose, or fitting.
If this pipe is connected to the overflow pan under your washer check to see if something is broken. If the water is steady it sounds as though a hose or fitting is broken. If it only happens when the machine is running, check the water level control as it may be stuck and causing an overflow within the machine.
Sometimes the discharge pipe is connected to other items that collect water overflows within the home. If you do not find anything wrong with the washer machine, check your AC system overflow pan if there is a system in the attic, or check on the bottom of your hot water heater as oftentimes there is a pan located under it. Any of these can be the source of the water.
AS KMS said, if the pressure relief is faulty the water will be hot and steamy. If that is the case, that is the cause. If the water is cool, or soapy its one of the overflow pans catching a leak on one of the items stated above.
or, if your AC is running it could be the condensation drain from your air conditioner.... if it is, it's just doing it's job. don't plug the line off w/ a cap or anything until you know what it is
run the line to a gully or ur septic tank