Tracking cookies (and pixel-tracking) are both methods that help collecting data around and about internet users. Cookies collect information and data on different formats, while pixels essentially do the same, but, apparently, with more variety in data collected on the user (and then sent to the pixel-creators server).
Advantages of tracking cookies and pixels (tracking, in general terms) could be that marketing can be targeted and be more relevant to the targeted subject. Disadvantages could be the ethical implications that the implementation of the tracking would provoke.