Skype has it's own version of SMS called Instant Messaging (IM's) that you can send and receive from other Skype users.
It is planned that SMS messages will be integrated into Skype along the lines of the now defunct messaging and Skype app, but for the time being this is still being worked on................................
All Skype to Skype communication (So video calls, calls and IM's) are free. You only need credit's or a subscription if you want to call landlines or mobiles from Skype.