Common Vulnerability Scoring System attempts to assign severity scores to vulnerabilities, allowing responders to prioritize responses and resources according to threat.
Scores are calculated based on a formula that depends on several metrics that approximate ease of exploit and the impact of exploit. Scores range from 0 to 10, with 10 being the most severe.
Common Vulnerability Scoring System LDAPWiki refers you to Wikipedia: Common_Vulnerability_Scoring_System