If you live in a country with 110V electricity, your electric accessories would not work in Greece where 220V (50Hz) electrical outlets are standard. You need to purchase an electrical adaptor to make sure all your equipment can be plugged into the Greek electrical outlets. An adapter (like the one pictured one the left) allows your plug to physically fit into the Greek power outlets.
Keep in mind that you will need more than one adapter, unless you want to charge your equipment one at a time. An adapter like this simply allows your plugs to fit into the Greek electrical outlets, it does not ensure that your appliance would work in the 220V current of the wall outlets in Greece.
Many electrical appliances have a transformer that allows them to adjust to different voltage, and some must be switched manually from 120V to 220V electricity. Check your manual to see how your appliance can handle different voltage. Even if your appliance does not have the ability to adjust to different voltage, you can use your own transformer.
A transformer is a device that reduces electrical voltage. If you do not use a transformer, you will damage your appliance if it is not designed to receive 220 volts.
To summarize, if your equipment can receive 220V (check your manual), buy a simple adapter. If your equipment is rated only for 110V (or anything below 220V), you need to purchase a transformer. If the transformer is not designed to be used in Greece, you need both a transformer (to reduce voltage), and an adapter to physically fit it into a Greek wall outlet.