In its simplest form, email marketing is just like direct mail campaigns. But instead of sending physical mailers to your leads, you're sending them content electronically.
Primarily, email marketing is used for keeping leads and customers informed about your company, educating them about specific areas of interest, and building relationships over time. You can launch lead nurturing campaigns, for example, that speak directly to a segment of your list and teach them about a solution to their main pain points.
Another common type of email companies leverage is an email newsletter.