Negative Keywords are an extremely powerful feature of Google AdWords. Fundamental assumption in search campaigns is that the performance of a group of keywords or search terms will repeat in a predictable manner over time. Some searches will convert higher than others and an advertiser should focus on these high converting search terms and bid higher for them. There will however be some search terms which might not convert at all or are not relevant to what you are advertising for. These should be added as negative keywords in your campaigns to avoid unnecessary clicks.
After careful analysis of different search terms, One can identify some which never converts. It is important to label only these keywords as negatives vs keywords which converts at a lower conversion ratio. It is highly recommended to continue bidding on these low converting keywords at a lower cpc to ensure the ROI is achieved. Once you have finalized the negative keywords, it is important to apply them across your campaigns. Adding negative keywords individually in a campaign or Ad Group won't guarantee that the ad is never triggered from your account.
For avoiding irrelevant searches, we recommend to create a shared negative list which can be applied across all your campaigns. This way the unwanted clicks will never trigger your ads. This strategy is advantageous over adding negatives at the campaign or Ad Group level as the maintenance becomes much easier over time. Keep in mind to add the negatives in the phrase match type instead of exact in most of the cases as you would want to avoid the variations of a bad keyword also. It is also important to add plurals of a negative keyword in the shared account as Google does not recognize plurals for negatives automatically.