Email marketing, at is core, is simply leveraging email in order to market directly to customers and potential customers. With this in mind it could be said than each and every email sent to a customer or potential customer falls under email marketing.
Typically email marketing uses creative emails (or simply text emails) to solicit a response from the recipient. It can be used to stay in contact with customers through something like a monthly email newsletter or to create new customers by presenting special deals of offers. Email marketing is also very effective in building brand loyalty and brand awareness.
Email marketing is usually done by sending a large volume of emails to a designated list. This can be a customer list or an outside list purchased strictly for the purpose of generating new leads or sales.