If you only read the Wikimedia project websites, no more information is collected than is typically collected in server logs by web sites in general.
If you contribute to the Wikimedia projects, you are publishing every word you post publicly. If you write something, assume that it will be retained forever. This includes articles, user pages and talk pages. Some limited exceptions are described below.
Publishing on the wiki and public data
Simply visiting the web site does not expose your identity publicly (but see private logging below).
When you edit any page in the wiki, you are publishing a document. This is a public act, and you are identified publicly with that edit as its author.
Identification of an author When you publish a page in the wiki, you may be logged in or not.
If you are logged in, you will be identified by your user name. This may be your real name if you so choose, or you may choose to publish under a pseudonym, whatever user name you selected when you created your account.
If you have not logged in, you will be identified by your network IP address. This is a series of four numbers which identifies the Internet address from which you are contacting the wiki. Depending on your connection, this number may be traceable only to a large Internet service provider, or specifically to your school, place of business, or home. It may be possible that the origin of this IP address could be used in conjunction with any interests you express implicitly or explicitly by editing articles to identify you even by private individuals.
It may be either difficult or easy for a motivated individual to connect your network IP address with your real-life identity. Therefore if you are very concerned about privacy, you may wish to log in and publish under a pseudonym.
When using a pseudonym, your IP address will not be available to the public except in cases of abuse, including vandalism of a wiki page by you or by another user with the same IP address. In all cases, your IP address will be stored on the wiki servers and can be seen by Wikimedia's server administrators and by users who have been granted "CheckUser" access. Your IP address, and its connection to any usernames that share it may be released under certain circumstances (see below).
If you use a company mail server from home or telecommute and use a DSL or cable Internet connection, it is likely to be very easy for your employer to identify your IP address and find all of your IP based Wikimedia project contributions. Using a user name is a better way of preserving your privacy in this situation. However, remember to log out or disconnect yourself after each session using a pseudonym on a shared computer, to avoid allowing others to use your identity.
Cookies The wiki will set a temporary session cookie (PHPSESSID) whenever you visit the site. If you do not intend to ever log in, you may deny this cookie, but you cannot log in without it. It will be deleted when you close your browser session.
More cookies may be set when you log in, to avoid typing in your user name (or optionally password) on your next visit. These last up to 30 days. You may clear these cookies after use if you are using a public machine and don't wish to expose your username to future users of the machine. (If so, clear the browser cache as well.)
Passwords Many aspects of the wiki projects' community interactions depend on the reputation and respect that is built up through a history of valued contributions. User passwords are the only guarantee of the integrity of a user's edit history. All users are encouraged to select strong passwords and to never share them. No one shall knowingly expose the password of another user to public release either directly or indirectly.
Every time you visit a web page, you send a lot of information to the web server. Most web servers routinely maintain access logs with a portion of this information, which can be used to get an overall picture of what pages are popular, what other sites link to this one, and what web browsers people are using. It is not the intention of the Wikimedia projects to use this information to keep track of legitimate users.
These logs are used to produce the site statistics pages; the raw log data is not made public, and is normally discarded after about two weeks.