You might be thinking of a voltage regulator or sometimes called a "Buck Transformer" - they'll convert DC input voltage into a different DC output voltage. They usually step-down (that is, from 12v down to 5v or whatever) and often need like... 1.5v of difference to work well, but some do step-up voltage and can take 12v to 18v or whatever, but at a reduced amperage.

Just be sure to get the right "size" (in watts or amps) of buck transformer for the needs of what you're powering. Getting a higher-amp/watt-rated buck is always better, having one that's under-rated can lead to a fire.