Domain appraisals are estimated values of domain names based on a number of factors. These factors may include domain age, traffic, revenue, brandability, market demand, keyword popularity, TLD, language, and more. On the Internet, there's two types of values that may be placed on domain names - it's wholesale or end-user pricing. Wholesale pricing is referred to the amount that can be expected from selling to a domain broker, or someone that invests in domains for a living. End-user pricing is referred to the amount of money that can be expected from selling to an end-user, or someone that actually plans to develop a site for the domain. Domain appraisals can either be automatically generated through an algorithmic program such as the ones you may find on a domain appraisal site, or be manually appraised by an expert domainer. Just keep in mind that auto-generated appraisals are inaccurate and flawed in many ways. Human appraisals are more accurate due to the consideration of semantics.