There are many certification programs for Scrum Masters. The most popular ones are PSM™ (Professional Scrum Master) from scrum.org and CSM™ (Certified Scrum Master™) from Scrum Alliance. A main difference between the two is that candidates must take classroom courses for the CSM exam, while it's not necessary for the PSM exam. That makes the PSM exam more popular among those who prefer to self-study. On the other hand, PSM is the most precise exam on Scrum, and reveals more about the knowledge of the candidate.
There are inconsistencies about Scrum concepts in the community. The content of this wiki is kept consistent with the PSM certification program.