Coolcryptovator posted a definition of plagiarism, but why not sticking with theymos' opinion on this?
Here is a theymos' opinion on plagiarism.
Plagiarism is what gets people permabanned, not just copying. Plagiarism is copying with the intent of passing the work off as your own. In essentially all cases, plagiarism deserves a permaban because it usually proves definitively that the person is here for the wrong reasons: to fill up space in order to get paid, not to actually discuss or contribute. If someone was able to convince us that they were plagiarizing just to eg.
impress people rather than to fill up space, then a lesser ban of a few months might instead be warranted. But this has never happened AFAICR. (Arguments based on plausible deniability aren't going to work; we don't need to
prove that you had the motive we see in your actions.)
If you treat posting as a job, a chore, then you must live in fear, since the forum is not made for you. In this case, you need to blend in as someone who actually cares, but plagiarism will immediately out you, and producing a mountain of useless posts will also eventually be noticed, if more slowly. If you do actually care, then this will be obvious in your posts (and probably your merit score), and you will have nothing to fear from moderators; even allegations of plagiarism will be doubted when seen in the context of your other posts.
in extreme cases could be copyright theft?
Plagiarism is almost always a copyright violation which could conceivably get the poster in a lot of trouble, but it's not a bigger legal issue
for the forum than anything else. (Using the forum to violate copyright is never allowed, though.)
when copying and pasting from the net can it lower google rankings? and internal copy and past could do the same thing?
That's not a particular concern of mine.
even memes may soon constitute copyright theft
Only in the EUSSR.
As long as it's not spam, I wouldn't worry about it 
