US homes use 120 volts at 60 Hz. Many other countries use 220 to 240 volts at 50 Hz. Plugs are also different.
Type A: two flat parallel pins.
Type B: two flat pins plus a round ground pin. Used for larger devices and computers.
US outlets take both. A plug adapter only changes the shape; it does not change the voltage.
Check your device's label
Look at the label on the charger or device, or the back of the power brick. Find INPUT.
"100-240V, 50/60Hz": works in the US with only a plug adapter. This includes almost all phone chargers, laptop chargers, tablets, cameras and many electric toothbrush chargers.
"220-240V" only: will not work properly on US power. Plugging it in may simply not work or may work badly. It needs a voltage converter (step-up transformer).
Devices that are a problem
Hair dryers, straighteners, kettles, irons, rice cookers, blenders, space heaters and other devices that heat or have strong motors.
These need a heavy, expensive converter, and some still do not work well (clocks and timers can run wrong because of 60 Hz).
Usually better: buy a US version. A basic US hair dryer, kettle or rice cooker is often $15 to $40, less than a good converter.
Do not plug a 220V-only heating device into a US outlet with only an adapter.
Use surge protectors for computers and TVs. A power strip is not always a surge protector; check the label.
Do not run space heaters on extension cords or power strips. Plug them straight into the wall.
Outlets near water (kitchen, bathroom, outside) are often GFCI outlets with "Test" and "Reset" buttons. If one stops working, press Reset. One GFCI can control other outlets nearby.
If many lights or outlets stop, a breaker may have tripped. Find the breaker panel (a gray metal box, often in a closet or garage), find the switch in the middle position, push it fully off, then on.
Buying a cheap "travel adapter" and thinking it converts voltage.
Using a home-country kettle and burning out the device or the converter.
Overloading one outlet with many heaters and appliances.