DC-to-DC Converter is a wide range of converter circuit and Buck Converter is a basic step-down converter, which means its output is smaller than input: Vout=D.Vin, which D is converter duty cycle.
Suppose that you need 3.3V with +/-100mV regulated source and you have unregulated volatge source which is variyng between 5-9V DC. The buck converter can produce 3.3V by varing its duty cycle in feedback loop. take a look at LM2574N datasheet. One major specification of DC-DC converte is their high effecincy, they do not generate so much heat.
Also in some application you may wish to have voltage source higher than input voltage source, or may need to generate a negative voltage from your positive source. These converter are boost and buck-boost converter respectivly. Some other second order converters also exist like cuk.
The discrete form of basic types is not suitable for master thesis, they are studied in past. But other field which is hot is resonant type converter, you can even do PhD thesis in this topic. Also integrated converter, feedback loop, ZCS/ZVS, SC stepup, is good.
ZCS: Zero Current Switching
ZVS: Zero Voltage Switching
SC: Switch Capacitor