This is a fair question and the answers can sometimes be confusing. Anyway, the first thing to do is to check what your instruction book says on the gadget that you want to use. Some devices have auto transformers that are able to detect and adjust to the voltage that they find on the end of the plug.
Others may have a switch which will enable you to select between 110V and 220V. You may find that some laptops may have a switch to select the voltage on the power supply that comes with them. You may also be able to buy power supplies for laptops with such a switch. I had an old Phillips shaver that had one of these switches. It worked great and I took it to Canada and the USA many times without any problem at all. However, one time when I was tired in the UK I switched the voltage selector instead of the the on off switch. Unfortunately when it connected to the electricity at the wrong voltage it fried the insides. So, even with a voltage selector switch you have to be careful.
One general effect with simple items such as light bulbs and electric motors is that when they are used on a lower voltage such as that found in the USA they will generally work slower or produce less light if it is a bulb. With more complicated devices you will find that they just won't work unless the voltage is high enough to match the 220V found in the UK.
The answer therefore is that if your appliance or device is not auto selective of the input voltage or doesn't have a selector switch then you will need an adaptor change the voltage from 110V to 220V