What is underapplied overhead?

1. Explain how overhead is assigned to production when a predetermined overhead rate is used.
2. What is underapplied overhead? When Cost of Goods Sold is adjusted for underapplied overhead, will the cost increase or decrease? Why?
3. What is overapplied overhead? When Cost of Goods Sold is adjusted for overapplied overhead, will the cost increase or decrease? Why?