In 2015, Facebook took a huge step towards to simplifying the advertising process by introducing the new Facebook Pixel. One pixel to replace all of them. The new pixel is now the only pixel you need. It has retargeting, optimization, and tracking all built in one.
Pixels are common across most advertising platforms. They're used to drop a cookie that will track visitors on your website so you can advertise to them later. This is called retargeting. Once you advertise to past website visitors, pixels can also be used to track their behaviour when they're back on your website. This helps you measure the effectiveness of your ads.
Facebook used to have a custom audience pixel for retargeting website visitors, and conversion pixels for tracking website conversions such as sales. While each advertising account had only one custom audience pixel, you could create many conversion pixels—one for each web page you wanted to track conversions on.