What is fan art? It's easily answered as art done by a fan or a particular subject, such as video games. I think the tougher answer is to the question "what isn't fan art?" As someone who does fan art from time to time, I find it upsetting to see those who copy and/or trace pictures done by the original artist declare such pieces as their own work.
At times, it's tough to tell whether or not a person's artwork is truly theirs or not without having a solid knowledge or recognition of the original artist's work. However, there are some that are obvious.
Community Gallery Doubles (or Triples...or Quadruples...)
There are many community-type galleries online often dedicated to a particular subject. Sometimes, one can look through their fan art gallery done by their members and see doubles, or even more copies of the exact same character, in the exact same pose, but with slight differences. The other peculiar thing is that it's done by different people.
Traced From an Animation Cell
This type of form is one where the character is all made of line drawings. Where the shadows are to be coloured are also outlined prominently.
Almost but Not Quite
The drawing is an exact copy of the original work, but the only difference is the colours. This still counts as copying.
A lot of fan artists in an Artist Alley at a convention do comission work right at their table. Some of them actually do their own take on a character, but there are those who just do a straight copy from an original. In this case, sometimes it's the person just making a quick dollar, but other times, it's what is requested of them by the client.
In a person's site with an art gallery, there may be rows and rows of beautiful fan art. But, when you click on their "original" art section, their artistic skill somehow drops drastically. Examples: The eyes could be completely lop-sided or the proportions are completely out of whack. Somehow, between the time you looked a the last picture of beautiful fan art and when you clicked on their original work, something went horribly wrong.
Swiping for the Lazy
Some people out there would actually swipe other people's fan art, post it on their site and claim it as their own fan art. This is even worse than those who copy other people's work and claim it for their own!
Could it get any worse? Indeed it can. All you have to do it take any of the types mentioned try to sell it to other people, claiming it as their own. Not only are they being disrespectful to the original artist by claiming it as their own, they are also trying to make money off of the "stolen goods".
Is it wrong to copy? I think that it's a yes and no. Yes it's wrong to copy artwork for the sake of trying to pass it off as one that you've done all by yourself. However, I think that it's fine to copy a picture for the sake of your own practice, as an exercise and nothing more. If you, for some reason, would like to show off this practice page to the world, make sure that you give credit where credit is due. Like a bibliography, mention its source and that it is your copy of said source.
I respect those who try to do their own fan art, even though it's not as perfectly done as one would like. At least they're honest about their work. It is a true fan art.
When most think of fan art, they restrict themselves to drawings/paintings and maybe 3d renderings. I think that fan art also includes things like arts & crafts dedicated to a subject, like sewing plushies, paper mache or even graphic or web design. As all fan art types, there are those who steal other's work.
Sewing patterns of plushies is something that have been stolen on many occasions. When using a pattern made by someone else, one should ask the creator of said pattern of the terms of its usage. Another thing also is, as with other fan art, to provide the origins of the pattern if it seems to be a general pattern for all to use.
The theft of layouts in graphic or web design is another problem. I won't go into too much detail about this as there is too much to cover. Generally, one should keep in mind that the designer of the layout has put much effort into it. Taking the exact layout and swapping a few pictures on it still counts as stealing. Granted, there are standard layouts out there, but there are those that deviate from the norm, which is what is being referred to here. Also, copying another's code can be considered as plagiarism.
One may wonder why the production of fan art is allowed, since its trademarks and copyrights are owned. Generally, it's free exposure for the companies and, of course, the money comes from the fans. There are instances where some companies completely disallow it, which often results in a backlash from the fans.
The Concept of Fan Art