This can vary from situation to situation but email marketing as Claire said before is an email that is sent out to existing customer or potential customers to get them to buy/use the service or product you are offering them in your email.
For example.
Hi, i am from such and such company,
Can i interest you in our latest product, the thingymawatsit 3000
please click here to place an order
These sorts of emails are sent out normally by the thousand but if you target the right customer base then they can be very effective tools for building a business.