The short answer is - it is complicated and that is a good thing.
The long answer:
We leverage our expertise on six critical elements (name, address, phone, email, business data and IP/device) of a contact record's data, cross-reference this with over 130 data points that can be derived from these elements and determine each element's authenticity and accuracy. We combine these results, along with some additional business intelligence, to deliver an overall quality score (0-100).
We also provide the quality score for five of the key elements, so you can see the impacts of a particular element (and potentially make additional business decisions on these results as well).
We have a number of great blogs that talk about Lead Validation, quality scores, and practical applications.
The best way to understand how it works is to grab a trial key and run some of your leads through the service to see the results. And then give us a shout to see how we can help make the most of the service for your needs.