Social media sites let users share their favorite websites and content with the rest of the world.   At its most basic sense, social media is a shift in how people discover, read and share news, information and content. It's a fusion of sociology and technology... Wikipedia

What effect does this have on my site?

Social media sites are a good way to drive more traffic to your website. They also provide more links to your website that search engines will find and count towards your link popularity.