DeviantART, like all other sites, suffers from a large number of unwanted material.
DeviantART is a growing community with active staff and admin doing so much to make everyone's time here a little more pleasant. But they also need help from the many deviants on the site to keep the galleries trimmed and tidy.

What is a Policy Violation?
A policy violation is a deviation that basically breaks the rules of DeviantART. When you join up, you agree to the etiquette policy of DeviantART, meaning rules must be followed during your time on the site. This can be as simple as not using the categorisation system properly, to submitting images that do not belong to you. If you see an image on DeviantART that you feel may break the rules, check out the Help and FAQ.
A Policy Violation is any deviation that can be deemed inappropriate for this website

Violations can come in all shapes and forms. Some are obviously more severe than others.

How can I submit a Policy Violation?
Every deviation on DeviantART has a "Report Policy Violation" button located on the page. It is found on the right hand side of the Artist's Comments, just below the box containing the thumbshare number. Clicking on it will take you to a page where you can submit the type of policy it violates and also a description of how the image violates policy. Saying you just don't like the image is not enough. There needs to be a VALID reason. As for the types of policy violation, there is a drop down menu with 5 choices:
Hate Propaganda
This includes deviations that promote or argues any themes including genocide, killing, or destruction of an identifiable individual, race, religion, nationality, ethnic origin, sexual orientation, gender or disability or which promotes inflicting on the group conditions of life calculated to bring about its physical destruction. These are serious policy violations. It is unfair to be prejudicial against anyone in such a way. If you feel an image can cause offense in the above ways, it is likely to be policy violation of this kind.
More Info:
FAQ #12: deviantART prohibits the submission of 'Hate Art' and 'Hate Propaganda', how is this defined?

Copyright Infringement
DeviantART does not allow the use of pre-copyrighted images to be used in your art. Use of stock images are allowed (There is a plentiful supply on DeviantART, as well as specific sites on the Internet), but images 'found on Google' are not valid images for use as stock.
Any screenshots from movies or games are not allowed to be used in any submitted art. It may not seem a big deal, but the rightful owner of the stolen images may decide to take legal action on the site, which will cause heartache for everyone.
Regardless of whether or not a deviant credits the original source, they are not allowed to display an image that does not belong to them.
This also extends to art ripping, which is art 'stealing'. This means that someone has taken the artwork of someone else (from DeviantART or anywhere else) and has claimed it as their own.
If you believe an image you see has been stolen from someone else, or is a copyrighted image, try and find the original image as best you can and copy/paste the link into the URL box on the policy violation page.
More Info:
FAQ #8: What does deviantART consider to be Art Theft?

Miscategorisation
A miscategorisation is an image that has been placed in the wrong category. This may not seem a major issue, but it is. If you can't be bothered to place art in the correct category, why should people be expected to view it? It also makes the system rater pointless. If everyone did it, searching by category would be useless. Choosing the correct category for your art allows people to search for it effectively and also keeps the place clean and tidy, aesthetically speaking.
The Gallery Directors will keep you all up to date on what does and doesn't fit in their gallery. Here is the latest Artists Relations Newsletter with a list of Gallery Directors and other helpful links.
Also, ^Ghouldaddy has a fantastic step by step guide on how to report a miscat in his journal Here.
For example, a photograph in the photomanipulation gallery is a miscat, as well as a picture fitting better into People and portrait photography than in horror and macabre

Nudity Violation or Pornography
Nudity is a lot different from pornography. A lot different. And Pornography, or anything that can be classed as pornography, is not permitted on DeviantART. This means sexual acts or similar. I don't want to have to spell it all out here, but the key points mean masturbation, sexual acts/positions, sexual toys, erections and masochism/sadomasochism are not allowed.
Also, the nudity of minors is prohibited on DeviantART for very good reason. This means people under 18 are not to be pictured nude at any point, or be any kind of alluring or sexual pose. This extends to an under 18 in lingerie or transparent clothing.
If you see an image that you feel is breaking these rules, report it.
More Info:
FAQ #565: You prohibit the submission of 'pornographic material'; what do you consider this to be?
FAQ #249: What is the policy concerning images depicting minors?

Other
This is where the policy violations that don't fit in the other categories go. This includes anything else that may not be allowed on DeviantART. For example, an image depicting actual self harm, or if a picture requires mature content.
More Info:
FAQ #220: What is Mature Content?
FAQ #284: What is the policy for submissions concerning self-harm and mutilation?
FAQ #7: What is the policy concerning photographs of corpses, cruelty or death?

Don't feel that someone you are reporting will be banned because of you. The admin on the site deal with each violation as they see fit, and it is extremely unlikely that they will be banned because of one thing. However, if they are banned because of one thing, it's a damn good thing they were reported, as it would be something pretty bad!
Being vigilant about policy violations is the key. The rules are important, so help is very much needed and appreciated by staff. Even submitting a policy violation every so often makes a difference.
When submitting a policy violation, don't feel you need to warn the owner of the deviation. It is often a nice gesture, but some people don't take it too well.
Reporting policy violations is for anybody that feels they can be mature and responsible enough to do so. It should include most of the people on the website, so do not think you are small and insignificant!

Now go forth and keep an eye out for work breaking the DeviantART rules.
You can be rest assured as your help will keep DeviantART on the rails.

Staff and Artists Relations: