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Here you can see the Lync Web App loading (it doesn't take very long for a very comprehensive and feature heavy application) and how it appears on the Lync client application on the desktop.
Though the Lync Web App is ideal for anyone ('guests') outside the company or enterprise to access a conference call or shared collaboration service, it can also be used for those within the company who are working from home or do not have the Lync client installed. Either way, it's merely another access point to gain entry into the service.
You will notice the Lync Web App is not only very similar looking to the client, but will also see that it performs nearly all of the functionality that is provided by the desktop Lync client. The only thing visibly not available to use is the VoIP facility, so one would need to dial in to the service, or have Lync call out to you.