Article Processing Charge (APC) Information

What is an article processing charge (APC)?
How much is the article processing charge (APC)?
What should I do if I cannot pay the article processing charge (APC)?
Why is there an article processing charge (APC)?
What do I pay if I am publishing Open Access in a journal that has page or publication charges?
Is tax included in the article processing charge (APC)?
Why has my APC increased?