As mentioned by seomarket, email marketing is only ideal in retaining interested visitors/customers, but not in acquiring new customers. That being said, retaining loyal customers is also an essential aspect of online marketing. Email marketing has continued to prove its effectiveness throughout the years, and the main reason for that is the fact that the inbox is a particularly personal environment, which makes the conversion rate on email marketing pretty high.
However, since your visitors have given you the permission to "talk to them" directly through emails, you have a responsibility to send them quality, high value content. Don't fall into the trap of sending soulless promotional emails, which makes you no different from a spammer. You might end up chasing away your most loyal customers if you are not careful with the way your emails are crafted.
Here's a great link to get you started on email marketing: http://www.copyblogger.com/email-marketing/
Hope that helps!