There are three main differences between Facebook posts and Facebook ads:
Unpaid organic posts are only shown to a small number of users who have liked/followed your Facebook Business Page. On the other hand, by using ad spend, ads can reach large numbers of users within or outside of your audience.
Facebook Ads use ad spend in order to reach large audiences. The amount of ad spend used has a direct impact on how many people an ad reaches. Unpaid organic posts do not cost anything to create but the reach for these posts is very low.
Facebook’s algorithm has significantly lessened the reach of unpaid content in recent years. For businesses, Facebook has essentially become a paid advertising platform. That being said, engagement such as likes, shares, and clicks can help increase the reach for both paid and unpaid content.