Not much is known about Yeti, other than that they seem to exist as companions, servants, and possibly protectors of the Snow Queen. In spite of their size and ferocious appearance, they have not been shown as dangerous. They are capable of human speech.
Yeti are large beasts (slightly above human-size in Solstice but much larger than the Snow Queen at the end of Snowbound) completely covered in white hair. Only their bare hands, feet, and their tiny eyes are visible.