Cell phones that support the GSM Wireless system (Global System for Mobile Communications) work in most places around the world these days. Most cell phone providers do provide GSM-capable phones, although you may have to call your service provider to have the technology “turned on” or “unlocked”.
In order to make phone calls or use data you may either activate an international plan or rely on WiFi. For the former option, call your cell phone provider to see what they offer and make sure to check about expected cost, options and limitations. Rick Steve’s post about calling and data-roaming is very detailed and helpful.
You might consider buying a local pre-paid SIM card to switch out – remember you will have a local number then. For more information about using European SIM cards in your smartphones check out Rick Steves’ blog post.