Skype to Skype video chats is always free, whether you're using the mobile app or the desktop program. To make a video call, both people must have the Skype program installed. Before you may video chat on Skype, you must first add the other person to your contact list. Audio calls between Skype users are likewise free.
It is not free to call a cell phone through Skype. To call a cell phone, you must have Skype credit on your account. Texting isn't free either; each message costs $11.2. If the person you're contacting has Skype and an Internet connection, you can save money by having him download the app to his phone and call it instead of the phone.