There has recently been a number of posts where people have included information from another website without linking back to it. While the web is meant to be a method of free trade of information, that does not mean that etiquette should be thrown out the window, and it doesn't always mean that the information on another website isn't copyrighted.
At the very least, quoting another website without referencing back is bad form and thus, rude. It can even be illegal, depending on the circumstances.
How you reference a source depends on how the information is presented.
Small Amounts of Info
If you're just presenting a small bit from a website, i.e. "The kuhli loach is an eel-shaped fish, elongated with slightly compressed sides, and very small fins." (which was taken from Wikipedia's "Kuhli Loach" entry) would be presented as something like "The kuhli loach is an eel-shaped fish, elongated with slightly compressed sides, and very small fins" according to
http://en.wikipedia.org/wiki/Kuhli_loach.
It's a good idea to do this even if you're not directly quoting a page. I will often include something like "Most of this info is directly off of the loaches.com site," if most of my info is coming from one source.
Larger amounts of info
If you're including more information, you may want to consider just posting the link. Copying and pasting an entire article off of another website, even if you reference it, is still plagiarism, and isn't very helpful. After all, anyone can search wikipedia for kuhli loaches.
In these cases, posting a simple link to the website is your best bet, if you feel that posting such a link is necessary. I often find it helpful to post the link and then give a general overview of the meaning of the article as it applies to the topic at hand.
There are more reasons to do this than etiquette:
1)You are supporting your argument. It's one thing to say "kuhlis like soft substrate." It's another to back that up with a link (or a few).
2)You are providing traffic back to a site that you believe is worthwhile (otherwise, why would you be quoting it?). If done consistently, this puts the worthwhile sites higher on the search engine rankings, making the internet a better place (it sounds cheesy, but it's true).
Pictures
Pictures are a special case. Do not presume that an image is free for use, even though it's up on the internet. It is always best to get permission to use an image before posting it. If permission can't be gained, simply post a link to the website where the picture is located.
Because we're not selling anything here, it's likely not illegal to post pictures without permission, but that doesn't mean it isn't incredibly rude. The photographer deserves to be allowed to choose how his or her image is used, and deserves credit (if credit is wanted) for its production.
There are a few exceptions to this. Anything posted on Wikipedia is public domain (they will only post images that are either in the public domain or were produced specifically for Wikipedia.) Likewise, there are sites that provide free images for public use. However, if you can't find something explicitly stating "these images are for free use," don't presume they are free.
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