Apart from being the title of one of the books in the Tiffany Aching series, the Wintersmith is exactly what his name suggests - the Smith, or creator, of Winter. He is the gales, blizzards, avalanches and frosts of the Disc. He howls in the eaves and drops snowfalls like hammers. But once, he fell in love.
Each year, when the Summer Lady fades and is replaced by the Wintersmith, the Dark Morris is danced, and in the spaces where the dancers are not, there dance the two immortals, just as they do when the Morris is danced at the beginning of summer. But one year the 111-year-old witch Miss Eumenides Treason took Tiffany up there and the dance filled her bones and she joined in. In doing so she inadvertently took the place of the Summer Lady, and then, being human, didn't fade away but stayed around. This so intrigued the Wintersmith that he took it upon himself to make her his queen.
As Tiffany-shaped snowflakes fell all around, and Tiffany-shaped icebergs loomed out of the mists on icy seas, winter began to last longer than normal, and grow ever harsher.
Finally the Wintersmith learnt a poem about what a human was made of, and constructed one so that he could woo Tiffany in its shape. However, the last three lines of the poem where what makes a human human and not a conglomeration of elements. These he could not understand and so could never truly be human.
The lines were:
Strength enough to build a home
Time enough to hold a child
Love enough to break a heart.
Without this understanding, he drew Tiffany to a winter palace where he crowned her, but with the awakening powers she is steadily learning about, she allowed herself to be the pivot in the see-saw that never moves and drew down the sun and pushed out its warmth into the Wintersmith, defeating him and allowing summer to return. Since these events, winters on the Chalk have evidently returned to normal.
It is undetermined how, if at all, the Wintersmith is connected to Jack Frost, although the latter may be one of his underlings in the ranks of elemental personifications.