It’s is the contraction for it is and it has; its is the possessive form of it. While we’ve all probably seen these prescriptions since we were children, confusion of the two forms is still fairly common. It's in a writer's best interest to take an extra moment to make sure the correct form is used.
- It’s [it has] been a long day.
- The cat licked its paws.
- It’s [it is] sitting on the kitchen table.
- The house is starting to show its age.
- Its fur is long and full of knots; it’s [it is] a pain to groom.