Tom Cagley’s blog explaining his take on the difference between Agile Coaches & Scrum Masters is really clear and I do agree, although in practice the roles and responsibilities may blur & can overlap quite a bit from time to time. As he explains:
So, while as a Scrum Master you might still also facilitate the organization in becoming more Agile, your primary focus is in the team or teams you fulfill the Scrum Master Role for. Anything you do outside of the team(s) flows from your focus on helping the team(s) grow in their Agile skillset.
Then again, as an Agile Coach, you are there to facilitate the organization first on their Agile journey, where coaching the Scrum Masters and/or teams follows from that.
It is never black-and-white, but I really like the way Tom wrote this down and clearly explains these often mis-used and confused titles.