What is the difference between MS and UC headsets?

I think the current accepted answer is a bit misleading, so I'll chime in a bit.

Indeed UC stands for Unified Communications. However, Microsoft Skype for Business is also a UC solution – as much of a UC solution as any other supplier's UC solution, such as Cisco's. So why do headset and speakerphone manufacturers produce generic UC and Microsoft compatible versions separately?

One is the button implementation. Depending on the specific buttons, the buttons will respond differently. Standard volume up and down will work as usual, but special buttons have different implementations (only the Skype for Business app responds to some).

Second is detection of the correct sound device. With the MS version, Skype for Business will always choose that device for the communication device. This is important for IT as the end user might be plugging in separate headsets for playing music etc., but Skype for Business will always default to using the correct headset/speakerphone (unless set otherwise).

Basically we went with MS certified in the Lync roll-out (nowadays Skype for Business) specifically for the correct device defaulting (ie. less IT support work), even though simple UC standard headsets work identically (if set up correctly).

UC stands for Unified Communications.

In the context of a computer headset, it means the headset is optimized for soft phones, or programs on your computer that connect to the office telephone system and pretend to be a real telephone. Functionally, the differences between an MS and UC certified headset should be slight if any.

Really, these certifications are for marketing purposes more than anything else. A headset is a headset is a headset. They have their strengths and weaknesses, but these are not due to the certification.

To add a bit more color to this topic. The certification is all about functional integration into soft clients. Microsoft, Cisco, AVAYA and others have a specific requirements to integrate a headset. So if you accept a incoming call with the buttons on the headset than certain commands need to be integrated. The difference between an MS and UC headset is that Microsoft doesn't allow other softclient integration into certified headsets, so call control won't work on a Cisco soft phone. A UC headset will work with all clients based on standard HID specification integration.