Many public wikis put all contributions under a free license. This helps to create a sense of community ownership and encourages long-term contribution. It is not generally necessary for a private or corporate wiki.
If you want to be able to use text from Wikipedia, and you want Wikipedia to be able to accept text copied from your wiki, you should choose Creative Commons Attribution-ShareAlike.
Wikipedia previously used the GNU Free Documentation License. The GFDL is a valid license, but it is difficult to understand. It is also difficult to reuse content licensed under the GFDL.