Explaining social media is fairly simple, it's obviously a form of online media that allows people to communicate with their peers and express their interests. Social media and search engine marketing are apples and oranges comparatively. They serve very different purposes and achieve their objectives in vastly different ways.
When a business is active on social media, it shouldn't immediately place an emphasis on directly driving conversions as it would through AdWords. It should instead focus on providing it's audience with valuable content (that can help improve their lives), through this methodical content marketing we can build brand authority and create relationships with a target audience. Your audience will begin to see your brand, not as a brand but as a social entity in it's own right.
Once your brands' social media presence is well established, direct response marketing can become a primary focus. Your audience will keep coming back to you, interacting with you and will hopefully purchase products from you, provided, you provide them with unique, valuable and helpful content.