Boosted posts are a normal social media post that you've sponsored with ad spend in order to extend its reach. There are less customization options than exist within Facebook ads.
With both ads and boosted posts, you can:
1. Specify audience.
2. Set a budget.
3. Choose how long to run the ad.
With ads, you can also:
1. Choose specific placements.
2. Set specific objectives.
3. Make creative decisions.
4. Leverage advanced targeting.
More specific information can be found here on Facebook's support page.