Skype for Business (previously called Lync in Office 2013) is an instant messaging client used with Microsoft Skype for Business Server or Skype for Business Online available with Microsoft Office 365 and Live@Edu.
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Pro Video conferencing
Lync allows real-time communication for up to 250 people at once. 5 of them will be able to use Full HD video.
Pro Collaboration tools
Pro Deep Microsoft ecosystem integration
Lync integrates well into the Microsoft ecosystem (Exchange/SharePoint/Office).