There's kind of three kinds:
- Free running oscillator: no current limiting, no regulation, dirty power in/out, requires minimum load. Typically <3W.
- Flyback, primary regulated (primary pulse height, or auxiliary winding sense): current limiting, poor regulation, dirty power in/out, requires minimum load.
- Flyback, resonant, or other: optoisolator regulated, current limiting, good regulation, not as dirty power in/out, no minimum load.
The latter is most like a general purpose power supply, just with other-than-mains input voltage. They're also the most expensive, but often the most worthwhile (e.g., RECOM and others' low capacitance, reinforced insulation modules, good for medical isolation, high voltage gate drivers, etc.).
Beware that almost all DC-DC (and AC-DC) converter modules are dirty as sin. If they carry EMC ratings (FCC or IEC specs), they're almost always meaningless. I've only seen one module that passed, once cables were attached: a medical grade module (15W, low isolation capacitance, better filtering than usual). The small ones (like the cubic 5-10W ones from XP Power, CUI and etc.) actually have no filtering onboard: you're expected to provide it. Line-side common mode filtering is typical, but you may find a secondary side CMC is needed as well, plus a suitably ingenious combination of Y caps between line, ground and output.
There are also HVDC to LVDC modules -- these work with an external rectifier or PFC stage, which can be common to many such modules. This allows you to share the AC input circuit, while generating a custom combination of output voltages. They're usually expensive, and needless to say, you provide your own filter externally as well.
Tim