From the Hubpages FAQ regarding the forums: http://hubpages.com/faq/#forums-violations
Signatures: Please do not sign your posts and especially do not sign your posts and include a link to your Hubs or another site. A link to your profile page is included with every post, and you can put information about yourself and links to sites you'd like to promote on your profile page.
If you're referring to hubs, some people do place a few sentences about themselves and their qualifications. However this is generally avoided - it can also be classed as duplicate content if the signature forms more than a certain percentage of your article. If you wish to tell people about yourself and your qualifications, you can do so on your profile page.