First, I'm assuming your phone has Skype installed.

1. If your phone has the capability to connect to wifi, you will be able to make free skype-to-skype calls and skype-rate skype-to-regular phone calls all using wifi.
2. You can call Skype-to-phone by buying skype credit, again if you have a wifi connection. Otherwise you run into having to pay for data roaming which is very expensive.