The Forum Signature is the third part of the fabled Internet Messageboard Trifecta of potential user-fail, along with the Username and Avatar. What do those tags at the bottom of your message really say about you?
Hey you. Yes you. Nobody cares about your forum signature. Not a soul. It's a meaningless juxtaposition of letters or images that is uninteresting to begin with and over time actively detracts from the enjoyment of reading a forum. The ubiquitous nature of these soulless message-codas on messageboards is similar to "Ads by Google", if they were designed by creatively crippled semi-morons and not marketing experts.
In general, the message relayed by a user's forum signature is pretty overt - "THIS IS A THING THAT I LIKE." With a hilarious Neil Patrick Harris quote omnipresent in your posts, maybe people can begin to think they're talking to Barney Stinson, and not someone boring or lifeless like you. This type of signature is what we'll call "Awesome by Proxy."
Other more rebellious types try to smash the comformist system by creating meta-signatures, such as "THIS IS MY SIGNATURE", or "If you can read this, you're too close" in tiny font. What these people don't realise is that everyone hates them.