Once my house reaches the temp set on the thermostat the fan outside keeps running until I turn it off what is wrong with the unit

The thermostat is not working right. It should tell the compressor to shut off once the temp has reached a certain degree.

Could be a defective contactor in the outside unit. Usually the contacts weld together causing the outside unit to run all the time. Do not allow the outside unit to run in this condition without the inside blower running at the same time since damage can result.