Skype for Business is included with your Office 365 subscription. It allows you to contact anyone in the business and send instant messages, make audio calls, video conference, send files and more.
By default Skype for Business is set to open on start up to stop this read the guide below. Be aware that if you stop Skype for Business from opening on startup you won’t be contactable until you manually start Skype for Business.
Stopping Skype For Business on start up: