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Could someone explain to me in simple terms what a 'signature campaign' is?

A way for an advertiser to create an incentive for greedy people to fill the forum with thousands of posts of completely useless nonsense.  The advertiser offers a small payment in exchange for the user to add the advertiser's ad to the user's signature space.  The user is required to create a particular number of posts on the forum so that the advertiser feels that the ad will get noticed.  Since the average greedy unknowledgeable user doesn't have enough useful and well thought out things to say to meet the advertiser's quota, they end up just spamming threads all over the forum with useless drivel.  Several of the regular forum users have taken to blocking (using the "ignore" link) all users with a signature ad campaign in their signature.  Therefore, by participating in such a campaign, you significantly increase the likelihood that helpful and knowledgeable users will never see anything that you post here.