What is "foldback short circuit protection" in a power supply?
A graph will illustrate the difference between classical current limiting and foldback.  

  

During normal operation of the PSU you move along the horizontal part of the graph: the voltage remains constant at variable current. Once the current exceeds the maximum allowed the voltage drops but the current remains (left graph), which may cause damage to the circuit, and also causes a high dissipation in the PSU itself.

With foldback current limiting, when the maximum current is exceeded the voltage still drops, but the current is decreased to a safe value. (right graph)  

(note: \$I_{SC}\$ means short-circuit current)  



edit (overvoltage protection)
Overvoltage protection means that the device suppresses spikes on the input voltage, so they can't disturb the output. This is often done by a VDR (Voltage Dependent Resistor, aka Varistor). The VDR is placed parallel to the input and will conduct if the input voltage exceeds normal values.