This is Part 5 of a 10-part series on 10 things we believe about search marketing. These 10 beliefs form our product design philosophy .
Ongoing negative keyword  discovery is an important part of optimizing your keyword research, keeping it "clean" and high-quality.
From a pay-per-click perspective, negative keywords are terms that might match your ad but which you don't want to bid on. For example, if you're running a PPC campaign for a stationery store, you might have an ad group for the keyword "notebooks." If you're using the broad match  option to catch long-tail variations like "bulk reporter's notebooks" and "back to school sale notebook paper," you run the risk of matching for unrelated search queries like "notebook computers."
To ensure that your ad doesn't display for such irrelevant searches, you need to designate "computer" as a negative keyword (in addition to "laptop," "PC" and other computer-related terms). Otherwise, you could blow your budget on useless impressions that will drag down your click-through rate (CTR)—and won't convert. This has a detrimental effect on your Quality Score  and return on investment for your whole campaign.
We believe that monitoring the search queries that trigger your PPC ads and regularly scrubbing your keyword lists of those irrelevant keywords is crucial for maximizing ROI on online advertising spend.
Negative keyword discovery and expansion is also beneficial for SEO. While you're cleansing your ad groups of unwanted keywords, you might as well eliminate them from your SEO research as well. This will ensure you're only targeting relevant keywords that will drive qualified traffic from both organic and paid search.