A boosted post and a Facebook ad often get mixed up.
What’s a boosted post?
A boosted post is a post that is already on your Facebook page’s timeline that you can apply money to in order to boost it to an audience of your choosing. This is the simplest way to advertise on Facebook. Boosted posts differ from Facebook ads because they are not created in Facebook Ads Manager and don’t have all of the same customization features.
What are Facebook ads?
While boosting a post is still considered an ad, Facebook ads are created through Ads Manager and offer more advanced customization solutions. There are many advertising objectives to help you reach your specific business goals and the audiences you care about most.
Where a boosted post may initially optimize for Page likes, comments, and shares or overall brand awareness, Facebook ads can optimize for app installs, website conversions, video views, shop orders and more.

When to use Facebook ads vs boosted posts
It’s important for any business to identify exactly what they’re hoping to achieve with an ad. For example, if you want audience engagement on your Page or to develop your brand awareness, boosting a post is a great way to maximize visibility and grow your audience. To create more advanced ad types and campaigns, use Ads Manager.
Sources: Facebook