It really depends where you are. In the south the climate is quite warm all year round. In the north the winters can be cold and wet, and in the mountain regions in the north there is good skiing in the winter. In central Italy the climate is generally mild in the winter although in the mountains (e.g. the Sibillini mountains) again there are ski resorts and the snow can fall very heavily even away from the mountains.
As most people travelling to Italy in the winter go to Rome, I have attached a link to a page with Rome's winter climate to this answer.
Related Q&A：
What is the climate in Italy in December？-Oh boy, in December, the climate in Italy can vary depending on the region. Generally speaking, it's pretty chilly. In the north, you can expect cold temperatures, and there might even be some snow. But in the south, it's a bit milder, though still not warm. You might need to bundle up with a thick coat, scarf, and gloves. It can be rainy too, so don't forget an umbrella. But despite the cold, the Christmas spirit and the beautiful decorations make the month quite charming!
