This is the answer I gave on Am I allowed to release a fan-made version of a Trading Card Game on a fansite?, which was marked as a duplicate of this one. I hope nobody minds if I re-post here as I think it mentions some points that haven't been raised here and summarizes a bit better:
I am not a lawyer, but as a software developer and occasional artist, I've investigated this a bit so I know what I can do to protect my work and also so I don't accidentally screw myself over. While it would help to know what country you're in, because different countries have different laws that may also affect you, I'll make an attempt at answering your question.
You'll likely run afoul of Copyright. The owner of Copyright over a certain work (and Copyright in most countries is implicit, and does not require it saying something is copyrighted explicitly) has full control over how and whether at all something may be distributed. This applies to any graphics in the game, any sounds. While it's OK in most countries to make a photocopy of something for personal use, and sometimes even pass it on to a friend, most courts will consider posting something on the internet (where it's publicly accessible) as redistribution. So unless all of this is in a closed, password-protected area, you shouldn't be doing this. It would be treated about the same as pirating software or movies.
Also, keep in mind that many countries have higher fines for commercial infractions. And in many countries (e.g. where I live in Germany) just having ads on your page or having a link to your company's (unrelated) store in the navigation will make it considered commercial (as you could be using it to draw customers there).
Names are usually covered under trademark law. So if anyone already uses that name, and it's not a generic term, you can not use that name. If they've registered the name as a trademark, they can cause even more damage more easily (but a trade mark doesn't have to be registered to let them sue you). So you can't use the others' name either. At most, you might get away with saying that your game is "in the style of" another game, but that's a risky proposition.
So in short: Don't do it. Just make placeholder graphics (or hire someone to do them), make up your own names. The rules are not protected, but even though you can draw your own graphics for a deck of regular French playing cards, the graphics of commercial decks being sold out there are copyrighted, unless they explicitly say otherwise or you can prove that the artist has been dead the requisite 70 years.
Note: There are fan productions out there that don't adhere to these rules and get by fine. That usually is totally due to benevolence on the part of the Copyright owners, you can't expect this in every case. E.g. Anne Rice was known for a long time to go after fan fiction web sites asking them to take down stories.
Update: Here's a recent case where Nintendo is requesting a takedown of a fan-game, though apparently they actually took graphics and sounds from original games, so this is a much more clear-cut case than most fan-games: http://www.eurogamer.net/articles/2015-03-31-nintendo-issues-takedown-notice-for-super-mario-64-hd-project
Update 2: Another interesting recent case is the recent hoopla over Star Trek fan films. It was surprising Paramount/CBS/Viacom/whoever else is involved right now let fans get away with this much for this long.
