You know how some ads seems to "follow you" around the Internet? That's retargeting, and it's one of most effective innovations in marketing.
The cool part is you don't need to be a huge brand to get in on the game and even if you have nothing to sell today, it's still considered a best practice to install the Facebook retargeting script (pixel) on your website.
This allows you to run Facebook ads to a Custom Audience of people you know have visited your website. That's a warm audience vs. a completely cold one or group of random strangers you selected based on interests or demographics.
This video shows how to install the Facebook retargeting pixel so you can start building your custom audience.