Email marketing is basically using emails to:
1. Market a product/service.
2. Generate internet traffic.
3. Create an awareness.
4. Stay up to date.
5. Remind customers of your company.
6. Convert more leads/sales.
7. Educate users.
8. Send out promotions.
And some others I might have missed. But generally, email marketing is a form of online marketing strategy. Not as fast paced as viral marketing but it's the same form of channel as those printouts you find in your residential mailbox.
Just that email marketing in some countries are more effective than printouts because no one reads printouts anymore.
Email marketing however doesn't equal to spamvertising. Like some companies who God knows gotten your email from where sends you something you're really not interested in. And worst of all, they don't even have an unsubscribe function.
So yeah...that's email marketing.
