Cynicism – an inclination to believe that people are motivated purely by self-interest.
Criticism – the analysis and judgment of the merits and faults of a literary or artistic work.
The most basic difference between these two are intent. One goes into analysis something objectively, and without bias. While the other will augment there opinion in a way that will benefit themselves or their organization first and foremost.
With criticism you can look and find faults in thing you love and enjoy. You can also find value and skill in things you hate or despise. You take your personal feeling out of the equation allowing you to analyze anything in a professional manner. These critical opinions will hold more weight in discussions about media and give more credence to the thing you say in the future.
With Cynicism you allow your bias to stem the flow of your argument. This can result in an opinion that can be dismissed and can undermine any legitimate points you make. This can also result in your opinion not being taken seriously. An example of this is IGN and their video games reviews. Because they don’t give poor scores on games that rightfully deserve it shows that they try to maintain a good relationship with game developers for early access to games. This undermines there opinions because you can no longer trust if a game is truly good.