Some Craigslist ads are free, and others require a small fee, but all ads are subject to the same rules and regulations. If Craigslist users notice that your ad is in violation, they can easily flag it using the links available in the top right corner of any ad. With the exception of the Best of Craigslist flag, flags are meant to identify problem ads on the site.
If your ad is flagged on Craigslist, it might mean that you put it in the wrong category on the site. Ads should be placed in the most appropriate category, such as Jobs, and also the most appropriate subcategory, such as Human Resources. Users who find your ad in multiple categories are also allowed to flag your ad for overposting.
Craigslist prohibits advertisements from containing certain content, and violators will be flagged. Content considered unacceptable includes threats against any person or group, pornographic language or images and impersonation of another individual. It is also forbidden to post ads that violate anti-discrimination laws -- for example, never post an ad that states a racial preference for job applicants.
If you paid for your ad and it receives a significant number of flags from Craigslist users, it may be put up for review by a Craigslist employee. As that are reviewed are not necessarily removed from the site, but may be if the content is deemed to problematic to remain on the site. Postings made in the site's forum are subject to the same policy.
If your Craigslist ad was free, a significant number of flags may be removed. Free ads are removed automatically, rather than being reviewed first by someone who works for the site. If you want to repost your removed ad, first double-check its content and make sure it goes into the correct category to avoid more future flags. According to Craigslist, a small number of removed ads are actually in compliance -- if you believe your ad is among them, reposting it in the same condition may not result in future flags.