A VoIP app (new word in the jargon, for application software) is synonymous to VoIP client. It is a piece of software that allows you to use VoIP on your computer and other devices like mobile phone or tablet PC, to make and receive phone calls.
Why Use a VoIP App? This question brings us back to why we use VoIP. VoIP has a lot of advantages over landline and traditional mobile telephony (GSM). The main advantage is the cost. With a VoIP app, you can make calls worldwide for very cheap, and most of the time for free. Besides, there are a lot of interesting features that enrich the communication experience. Included are the benefits associated with unified communications. VoIP apps are also a fundamental element in cloud communication systems.
Take a look at this video for a quick demo of the soft client.