The difference between organic social and paid social is simple. Organic social content is posted for free. Paid social is content you’ve paid to appear in newsfeeds.
Organic social posts will show up on the feeds of people that already follow you.
Paid social posts will show up in the feeds of the audience you are targeting. You can target audiences based on demographics, location, interests, and more.
For how to build a paid social strategy see here: How do I create a paid social strategy?