The t&p valve is a common source for water to start leaking from an old water heater. The t&p valve is located either on the top of side of the hot water heater. The tp is a relief valve that can be used to relieve pressure from the water heater. It is important the the tp valve be extended to within 6″ of the floor. If water comes out of this valve it will very likely be scolding hot unless the water heater has not been working.
If your t&p valve is leaking it does not necessarily mean that your water heater needs to be replaced. The valve is universal and can be removed and replaced from your old unit. If you are going to replace this valve yourself, make sure that you have drained the water out of the tank or you could be scolded.
When the t&p valve starts to leak, it could be an indication that there are more serious issues with your homes plumbing. Often when the pressure is too high it will cause these valves to fail. The pressure inside your house should be regulated if it is over 80 psi. There are two methods of dealing with the pressure surges. The first is a PRV or pressure reducing valve that will connect to the main water shut off. The second preventative method is to add a thermal expansion tank to your system. In many cities this is required by code.