Refers specifically to one's husband's father. For wife's father, use 義父 (ぎふ) or 妻の父. The reading しゅうと is standard; しうと and しいと are obsolete.
See also: 姑
My father-in-law gets up early every morning and tends the garden.
Living with one's father-in-law can be challenging at times.
Since getting married, I've been living with my father-in-law.
I chose my words carefully to be considerate toward my father-in-law.
Kana spelling is common in everyday writing, especially when the kanji is considered difficult.