Userboxes, are small tags that can be placed on your User page to denote affiliations, hobbies, etc. They can be either public (in the Template namespace), or private (in the User namespace).
List of existing User Boxes can be found Here. You can see The hairy bear and Scafloc's userpages to see examples of what they look like.
Using a user box is as simple as adding the template to your user page. For example:
You can also add Userboxes from Wikipedia list of humorous Userboxes, this way:
Alternatively you can create userboxes of your own using the Template:Userbox
Note, that there is no limitation, on who can add what Userboxes to his page and it would be nice if you would dedicate something to your fellow editors. Also the Userboxes dont really mean anything, it's just for fun and to connect the active contributors together.