What is the difference between sign up and enroll ?Feel free to just provide example sentences.

Enroll: register in an institution (like a university) or a class.
He enrolled in medical school.
She tried to enroll in that class but it was too full.

Sign up: to join or register (it's less formal than enroll)
He signed up for a Gmail account.
She signed up for the pottery class.

(You can use "sign up" more broadly and in many of the same sentences where you would use enroll: "You should sign up/enroll for that class." But, generally, you enroll in an educational institution and you sign up for something like an account online.)