A Facebook pixel will trigger a HTTP request to a Facebook server end point containing information about the viewer. The data in the pixel will be processed and the Facebook server will try to match it with a Facebook user.
The matched users will be added to the relevant website custom audiences. You can filter the pixel traffic with filtering rules against the url and other custom data passed in the pixel.Facebook Pixels
The pixel events are also crucial part in setting up Facebook Dynamic Product Ads, one of Facebook's best re-marketing tool.Dynamic Product Ads
Facebook Pixels work with most tag managers too. Read this about how to set it up.Tag Managers
The Facebook Pixel Helper is a troubleshooting tool that helps you to validate the Facebook pixels placed on your site. You can read the guide document on our developer site for more details.