A DC to DC converter is a type of switching regulated powersupply whose input is a DC voltage, it then switches that rapidly to make AC, which is applied to the primary of a transformer, the transformer then either steps up or down the voltage as desired, the AC from the secondary is then rectified and filtered to provide the output DC voltage. Regulation circuitry measures the output DC voltage, continuously adjusting the switching circuits on the input to keep the output constant.
Most switching regulated powersupplies are DC to DC converters with a simple unregulated rectifier and filter to convert the input AC to DC. Usually it is called a powersupply when it provides more than one output DC voltage or if its input is AC; whereas it is called a converter only if it provides just one output DC voltage and its input is DC. But most of the circuitry is the same in both cases.