Negative targeting is a technique used for PPC search advertising and display advertising.
On PPC search platforms, negative targeting refers to the use of negative keywords for ensuring that customers who do not belong to the target will not be exposed to an ad. Negative targeting prevents useless ad impressions and clicks and preserves quality score. It is a precious tool for PPC campaign optimization. For more details see negative keyword.
For display advertising, negative targeting is the process of defining specifically a segment of audience which must not be exposed to the campaign. When the feature is offered by an adserver - or adexchange, DSP, etc - negative targeting is done with the same targeting options available for classic targeting.
For instance, if an advertiser does not want deliver his campaign in a specific US State, he can use negative geo-targeting for this state. It is faster than selecting the 49 other States.
Negative targeting is also used in site retrageting campaigns. In this case, an exclusion pixel is needed.