Most electrical devices that travellers take with them (camera and MP3 chargers, electric razors, and laptop computers) will accommodate a wide range of voltages and frequencies. However, these devices usually come with the plug type that is common in their home country and an adaptor is sometimes needed to use electrical plugs in other countries. Some overseas plug types will accommodate foreign plugs (especially in better hotels and conference centres), but you never know, so it is best to bring along plug adaptors specific to the countries you plan to visit.
Always ensure that your device can accommodate different voltages and frequencies by reading the back of the plug, power supply, or on the side of the device in question. A good sign is something like 'Input: AC 100-240V, 50-60Hz'. If your device will not accommodate a wide range of voltage and frequencies, then consider purchasing a converter or transformer to allow you to use the foreign electricity in Greece. Lower voltage devices sold in North and South American plus Japan (100-127 volts) that are not rated for higher 220-240 volts are a fire hazard if plugged into a higher voltage plug. Devices used with a different frequency (noted in Hertz or Hz and measured as cycles per second) than recommended may give incorrect readings or overheat. This should be of particular concern in timing devices, such as alarm clocks as they keep time based on a particular frequency. Some timing devices, especially those sold in Japan, will have a frequency switch on the bottom to change between 50 and 60 Hz.
If you are using an unstable power source on your travels, especially in remote locations with generators or on boats, also consider bringing a surge protector as voltage spikes and frequency changes are more likely to damage your valuable equipment.