A signature is a piece of text and wikicode which identifies you as the author of a block of text. It contains an automatic link to your user page, making it easy for others to learn more about you and communicate with you. The date contained in the signature also indicates how current the discussion is and the order in which comments were made.
On talk pages and forum pages, signing your name helps others to keep track of discussions, and to understand who holds which opinions. This is considered good "Wikiquette".
Generally, your contributions to articles, help pages, policies, and other content pages should not be signed.
How do I add a signature?
To sign your name, type four tildes in a row:
In Source Mode editing, the signature button ( or ) above the edit box can also be clicked to leave four tildes.
You can change this signature to point to another wiki, or to your talk page. For example, if you are signing a comment on Community Central, but you are most often on the Doom Wiki, you may want a link to the talk page you visit most often. In this case, you want your signature to be:
Note: It is recommended you link only to your user talk page on your home wiki, and leave the link to your user page pointing at the wiki where you signed a comment. This allows users to access your local user page and to view your contributions list for that wiki.
Note: Keep in mind that a link to the current page will not be displayed as a clickable hyperlink (e.g. the link to your talk page in your signature on your talk page).
Check with your wiki's community for whether there are any rules about styles in signatures.
What about anonymous users and signatures?
If you chose to edit Wikia without logging in, the tildes will be converted to your IP address. In such cases, it may make more sense to manually sign your posts with a pseudonym or tag such as --anon. (Note that choosing not to sign with tildes does not keep your IP address private, since the IP still appears in the page history.)
What about signatures in other character sets?
If your signature is in a character set not in wide usage on the wiki you are signing on, it may be easier for users there to recognize you if you add a version of your name in the character set of that wiki.
In addition to difficulties users may have in remembering and using a name in a script which is foreign to them, characters from many scripts (such as Chinese, Hindi, or Georgian) may display as boxes or mojibake (garbage characters) for users without the proper fonts installed.
In these cases, you might want to add an additional nickname to your signature that is more understandable on that wiki. For example, you could use something like [[User:快樂|快樂 (Happiness)]] or [[User:快樂|快樂 (Felicidad)]] in the Custom signature: field.
Where can I find more help?
The English Wikipedia has various pages relating to signing comments, and to making comments in general, which may help to guide your project's development of signature and talk page policies.