A Signature campaign is what you call a way of promoting an ICO or Gambling site or mixer service, here in btc talk forum, by hiring members to post regularly. It's called a signature campaign because the participants are made to wear signatures in order to advertise. The participants are usually paid in btc.
Bounty is used for campaigns which promote an altcoin in tokens. Usually there are many ways of promoting available in a bounty campaign. There's Twitter, Facebook, Youtube, Reddit and even Blog campaigns. A signature campaign may be done under a bounty but if it's paid in tokens then it will still fall under the category of a bounty campaign.