Most portable equipment tends to be dual voltage, including many chargers for phones etc.. but obviously check first as plugging in a 110V device into 240V will cause permanent damage.

Power frequency also differs. Most countries use 50Hz, but the US to be different uses 60Hz.

Not sure why the US mainly has 110V, maybe they thought it was safer. Often in the UK power tools used on building sites are 110V.

The other thing that is different is video standards. Most of the world uses PAL, the US uses NTSC. Almost all the TV's in Europe will support NTSC playback, also DVD players are often 'codefree' and play DVD's from all regions. I don't know if there is a difference in the HD formats, for once I think they are the same.