Roamer works by linking your regular mobile number to a local SIM card while you are traveling (you need to obtain a local SIM card yourself). In case you cannot get a local SIM card - don't worry, Roamer would still work over WiFi connection. In general, any SIM card in the country of travel works - either voice or data-only type (make sure to enable Calls over Internet in Roamer with data-only SIM cards).
You enjoy local 3G/4G data at cheapest rates (with a local SIM) whilst receiving/making calls with your regular number.
Roamer supports most of the countries in the world as travel destinations, however roaming-free incoming calls are not available to users of all countries. Even if your country is on the list, sometimes it depends on your SIM plan - for example prepaid SIM cards often do not offer call forwarding, thus making it impossible to enable Roamer incoming calls. However, if your plan does not offer call forwarding you can still make calls with your Caller ID when abroad.
Besides roaming, you can always use Roamer to make international calls - even when you are home! Just tap a contact from the address book or return a call from History.