Electrical outlets heat up because of faulty or short-circuited wiring or an overloaded circuit. Poor wiring doesn't allow the wires within the outlet to make contact with the terminals of the outlet or the appliance plugged into it, increasing the electrical resistance.
Electrical resistance occurs because electrons traveling in a circuit generate friction as they bump into one another. At points where there is little room for them to move, this friction increases and so does the level of heat. Short-circuited wiring, or wiring that touches a surface that allows the energy to deviate from its designated path, also increases resistance.
Overloaded circuits can also heat up outlets. Each electrical circuit is designed to hold only a certain level of current at once, and exceeding this puts strain on the internal wiring.
Another factor is improper grounding. Sometimes, a circuit can experience a power surge. Grounding is the process of routing excess power to a rod buried in the ground. If the outlet is not connected to a grounding rod, the extra energy has nowhere to go.
Do not use electrical outlets that are hot to the touch, and call an electrician to repair them. If possible, cut power to the circuit containing the outlet until it can be repaired.