Basically the use of email to promote products and/or services. But a better email marketing definition is the use of email to develop relationships with potential customers and/or clients.
The huge advantage of email over social media is that prospects and customers are more likely to see an email than social media. Just posting something doesn't mean that everyone you want to see your message will see it. Your post might not even show up in your targets' social media streams. However, an email will sit in the inbox until it's read (or deleted).