It is likely one of three things:

1. If it is a single pipe behind the water heater, like in the photo above, then you have an abandoned discharge pipe for a water heater TPR (Temperature and Pressure Relief) valve. The other end of the pipe  usually comes out at an exterior wall of the home, pointed down, and near the ground.  
      Abandoned piping like this should ideally be removed and the openings in both the interior and exterior walls repaired; but, at a minimum, the pipe should be capped at both ends to eliminate a route into the house for insects or vermin.
      The TPR discharge pipe was originally required by the building code to extend from valve to near the ground on the exterior of a house. But a change in the building code a few years ago allows the pipe to go to within 6-inches of the floor next to the water heater, unless the waterdrainage there would cause structural damage. Then the TPR discharge piping will terminate just above a catch pan under the water heater with a drain to the exterior.
       As an alternative, a new water heater can be connected to the existing TPR drain piping when replacement is necessary. But a requirement that the discharge piping drain completely by gravity with no trap (uphill section that would retain water) makes this solution difficult when the TPR valve on the new water heater is below the existing piping into the wall for the old water heater.
   
   
2. If it pipes are under a sink, behind toilet, or more than one abandoned pipe behind the water heater, then the home has had the water supply piping replaced. The original piping, usually galvanized steel or copper, has been sawed off near the wall and left in place, like in the photos below. One set of abandoned pipes below is open and the other set has been sealed. Again, the best solution is removal of the abandoned pipe and repair of walls.
   
   
   3)  If the pipe is low to the ground near a water heater, range, or HVAC air handler, then it is probably gas piping for a former gas appliance that has been converted to electric. When the pipe is active and simply has a shut-off valve at the end, but not a threaded cap, it is a safety hazard and should be permanently sealed if not completely removed.

   Also, see our blog post What are the pipes on my roof? 

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Can galvanized steel pipe still be used for new water lines in a house? 

• How can I tell if I have cast iron pipes in my house? 

• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

• What is the difference between green and white sewer drain pipes?

• Is a washing machine drain hose required to be secured at the standpipe? 

• What are the code requirements for plumbing vent terminations?

• What are the code requirements for layout of drain piping under sinks?

• What causes a gurgling sound when a bathtub or sink drains? 

• What is a "combination waste and vent" in a plumbing system? 

• What is a building trap?  

• What is a dielectric union? 

• What's that powdery crust on the pipe connections at the water heater? 

• Can you connect CPVC pipe directly to a gas water heater?  

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.