Social media is a subset of electronic commerce, it includes various social media. Social media is used for online interaction and also for online buying and selling. Firstly, social commerce was introduced in 2005 by yahoo. Social commerce includes customer ratings and reviews. Social media impacting the e-commerce industry very much.
Some social networks actually allow you to sell things through them, to varying degrees. Very recently, Facebook has introduced an online store functionality that will allow customers to purchase items straight from your brand's Facebook Fan Page. Pinterest is also a huge player when it comes to social commerce - they allow you to put price points and link directly to product pages when you pin your inventory. This is huge for online sellers in a lot of ways. Did you have a more specific question? Asking "what is social media commerce?" is super broad, and might make it tough for us to give you answers that actually address your specific problem or issue.
Using social media is also termed as Social Commerce, which involves social communication, buying and selling, promoting projects or company etc on social platforms.