Scrum is the most popular Agile process that is used for a complex domain/product.
Scrum is not a step by step process that when followed leads to a software product rather it is a framework to manage software development work.
When followed every organization can have their own specific version of SCRUM.
Scrum is the best process where requirements keep on changing for a complex domain. Scrum can be used for a simple project but the process would be an overkill.
Scrum is also not suitable for a complicated domain where business is dominated by experts.
Essential Scrum: The Most Popular Agile Process - By S. Kenneth Rubin.