Are M+S tyres winter tyres?
Not necessarily. This name us also used on tyres for 4x4s and on all-season tyres.
On the same topic
M+S marking on the sidewall means "Mud & Snow". All winter tyres carry this mark, but the reverse is not true. All tyres carrying this mark are not winter tyres.
The mark M+S is the result of autonomous classification by manufacturers. It can also be found on all-season tyres and on tyres for 4X4s.
It is known by the acronym 3PMSF (Three Peak Mountain Snow Flake).