The regular Skype app allows free or cheap voice calls and video chat via the Internet, whether the data goes over 3G or WiFi is up to your phone settings.
Skype WiFi on the other hand is a special app that makes it possible for you to use your Skype Credit (if you have any) to pay for WiFi access in hotels/airports etc, where the WiFi is not free. You can then use the Skype WiFi App to pay for this access instead of a credit card or similar.
As per PC World, the service is quite affordable--especially when compared with the sort of prices that Wi-Fi hotspot services typically demand. The best part of Skype WiFi is that you it is billed by the minute and you only pay for the minutes you use instead of buying 15 minute or one hour blocks that you may not use.
Make free phone calls with these free internet phone services and programs shared by Thebalance.com.