When you resume or start Skype, it will remain signed in to your Skype account. Skype for Windows Phone8 also allows you to switch the active Skype account, for example from your business account to your private account or your “offline” account.
You can’t turn Skype off once you sign in for the first time and supposedly there is no need to because it is so well integrated into the os that it does not use up additional power by being left on. I have mine installed for a while now and have not noticed any effect on battery life so they must be right.
When I need to be offline or “switch off” the Skype, I’ll activate my “offline” account.