It's almost like direct mail and/or sending a print newsletter - as long as the person on the receiving end ASKED to receive that content.
For example, blindly sending 100 letters to different households in an attempt to get a response ... is generally how direct mail is performed. However, with email marketing, 100 people will have actually given you their email address ... wanting to hear from you.
So, email marketing is similar to direct mail in that you're sending an email/letter straight to their inbox ... the difference is that email marketing is more permission based.
So, if you think of email marketing like that ... the next logical step would be to get a list of email addresses.
And, before getting into that ... you will want to pickup an email service provider (ESP) that will be able to keep track, organize, and automate your emails. I personally use and recommend both ActiveCampaign and MailChimp.
Once you have your ESP, it's time to grow your email list. This is normally accomplished by adding a signup or "optin" form to your website and by offering some form of incentive (Lead Magnet) in exchange for the individual's email address. This can be a coupon, checklist, video course, eBook, etc.
From there, you'll be able to send emails to your list, building a relationship and promoting products.
And the good part about email marketing is that it's relatively cheap to perform. You do not need to purchase stamps, paper, etc in order to spread your message which makes it much easier to build a relationship and keep your business in the front of your customers minds.
So, that's just a general overview of how email marketing works. Of course, there's a lot more to it than that, I was just trying to keep it simple enough that even my mother would be able to understand!