The electrical current in Greece is 220 volts, 50 cycles AC. Wall outlets take Continental-type plugs with two round oversize prongs. If your appliances are dual-voltage, you'll need only an adapter; if not, you'll also need a step-down converter/transformer (United States and Canada).
Consider making a small investment in a universal adapter, which has several types of plugs in one lightweight, compact unit. Most laptops and mobile phone chargers are dual voltage (i.e., they operate equally well on 110 and 220 volts) so require only an adapter. These days the same is true of small appliances such as hair dryers. Always check labels and manufacturer instructions to be sure. Don't use 110-volt outlets marked "for shavers only" for high-wattage appliances such as hair dryers.
Steve Kropla's Help for World Travelers. This Web site has information on electrical and telephone plugs around the world. www.kropla.com.
Walkabout Travel Gear. This Web site has a good coverage of electricity under "adapters." www.walkabouttravelgear.com.