Social media marketing (SMM) (also known as digital marketing and e-marketing) is the use of social media—the platforms on which users build social networks and share information—to build a company's brand, increase sales, and drive website traffic.
Here is a very thorough report from Wikipedia explaining this type of marketing.
What is a good ROI for social media advertising?
As a general rule, businesses should aim for a return on investment (ROI) of at least 3:1 for social media advertising. This means that for every dollar spent on advertising, the business should earn at least three dollars in revenue.
There are a number of free videos on YouTube which profess to tell you how to do effective social media advertising. You can also find people who claim to be experts at this type of marketing on sites like fiverr.com or Upwork.
Some of the most used social media sites are: Facebook, YouTube, WhatsApp, Instagram, and for business -- LinkedIn.