Broadly speaking, email marketing is the dissemination of marketing messages via email to a group of customers, potential customers, or other interested parties. These can take the form of direct solicitations for purchases, branding and informational newsletters, purchase follow-ups, abandoned cart emails, reminders, and more.
They can be sent via homegrown email lists or through a service that rents lists, and they can be segmented to specific subgroups based on any number of options, allowing the sender to narrowly target messages for the best response.
Email marketing also boasts many of the same great features found in other digital marketing channels: easy campaign tracking and measuring; simple methods for testing design and messaging; and low cost to implement.