I have to expand on Braiam's answer by pointing out that blithely copying and pasting in stuff from other websites (even in the tag wikis) is unacceptable. Our Terms of Service stipulate in section 3:
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license.
By posting, you grant a license to content. If you don't own that content or can't justify its use as fair use, you've fraudulently given Stack Exchange a license for something you had no right to in the first place. It's wilfully illegal and puts Stack Exchange in a bad position (that's why these clauses exist in the first place).
We can't allow behaviour like that.
Regarding content already on Stack Exchange (Ask Ubuntu, Stack Overflow, etc), all the content is licensed under CC-BY so you can copy as long as you link to the post and the profile of the user who posted it.
An important distinction is that this isn't the same with all content on the Internet. CC-BY is an exception to the general rule. Implicitly all content is "all rights reserved" unless it says otherwise. Attribution does not get you past that.