Facebook says a boosted post originates as a post to your Business Page’s timeline; a Facebook ad originates in Ads Manager. 

A boosted post is not as customizable as an ad and has fewer “placement” options.

To us though, there’s no difference. Both boosted posts and ads from the Ads Manager appear in your audience’s Newsfeed and can step on each other, negatively affecting our ads’ performance and Growth X results.