(1) Skype has video calling where as Viber at the moment doesn’t have video calling.

(2) Skype Client Software could be installed on any compatible device and same username password pair can be used to login and make calls. Where as in Viber, it currently supports for iPhone only and promising to release Android and Blackberry versions soon.

(3) The main difference in Viber is , you don’t need to run the viber application to receive calls whereas when you receive a call it launches the application and rings. It’s kind of, viber server sends push alert to viber application when a call is received. In VoIP context, before viber server sends the signalling (Probably SIP) it sends a push alert to launch the application and sends SIP invite to initiate a call.

(4) Address book synchronization is possible in Viber but not in Skype.

(5) Skype is using their propriety CODEC , which could use minimum data possible with high quality.

(6) IM, SMS, Skype Out, Skype In is possible with Skype but at the moment it’s not possible with Viber but Viber promissed to introduce Free SMS and probably will introduce Viber Out (A-Z Termination) in future.

(7) Viber and Skype uses your monthly mobile data plan or could be used in WiFi.

(8)Viber and Skype give good quality carrier grade voice

(9)Viber announced to release Free SMS among users whereas Skype doesn’t have this feature but Skype has regular SMS and IM for realtime chatting.