A switching-mode power supply (SMPS) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Switching regulators are used as replacements for linear regulators when higher efficiency, smaller size or lighter weight are required. The main advantage of the switching power supply is greater efficiency than linear regulators. The downside of them are that their circuits are more complicated than linear power supplies and their switching currents can cause electrical noise problems if not carefully suppressed. If the SMPS has an AC input, then the first stage is to convert the input to DC.
Here are some tutorial videos on switched-mode power supply technology:
How Does a Switching Power Supply Work 1 (schematic, explanation, example, modifications)
How Does a Switching Power Supply Work 2 (measurements)
How Does a Switching Power Supply Work 3 (CCM vs. DCM)
Simple 12V 1A LED Power Supply – with schematic and transformer autopsy
12V 5A LED switching power supply – with schematic
12V 5A Switching Transformer Autopsy – How it’s constructed? Is it safe?
Howto repair switch mode power supplies #1: basics, and block diagram of a PSU
Howto repair switch mode power supplies #2: Stand-by circuit and its functionality