When I receive a message I don't want the sender or a preview of the content be shown on my lock screen. Some apps (like iMessage and Mail) allow you to set this in iOS' Settings -> Notifications. Other apps (like Viber) allow you to set this internally from their settings. But there are apps (like Skype), for which I don't see a way to set this. Is it possible to set this globally for all apps?
It looks like every app determines what to show on its notifications; I don't see a global switch.
You can, however, turn the lock screen notification off.
Find it on Settings > Notifications > [App], look for "Show on lock screen" and turn it off to disallow the app to show its notifications when the phone is locked.
Skype does not have the "Show Previews" switch available at the Notifications settings for Messages –and I can't find anything about that subject on the support site, but plenty of people asking the same with no response– so it appears that each app controls what it's shown on the notification.