Everyone who is logged in can leave a signature - useful to sign a contribution, a personal remark on someones user-page, a statement on a discussion page, and most of all useful if one wants to leave ones name on a list of participants.
To create such a signature like
--Olaf Simons 11:56, 11 May 2007 (CEST)
you write four tildes: ~~~~. As soon as you press "save page" these signs will be converted into a link to your user page with a note of time and date. You can check the authenticity of any such signature with a check of the respective "page history" on which all edits (also the one which created the questionable signature) are connected with the accounts who produced these edits.
Three tildes ~~~ will be converted into a User-name link like Olaf Simons