You select your contact and Cloud Sim calls you and then connects you to the number you are calling. Callback should be used in countries where Local Access is not available.
Callback calls are slightly more expensive as there are two parts to each call. The app requires a very small amount of data to initiate the call.
If you are using Callback with a bundle you will need to buy a bundle both for the country you are calling and the country you are calling from. Otherwise you will be charged our per minute rates.