Prednisone is a glucocorticoid used as replacement therapy in patients with adrenocortical deficiency, and also as anti-inflammatories in people who have no deficiency. They are used after injury, for rashes, arthritis, tendonitis, and dozens of other things.
Side effects depend on the person, the dosage, and the length of treatment. Mild irritability, insomnia, etc. are common. Here is the prescribing information. Please let me know if there is anything specific you have questions about.