What Is Severity and Priority?

severity describes seriousness of the defect with respect to functionality where priority describes importance of defect to solve with respect to customer.
Example:

Suppose we identify two bugs

-The client logo is not appearing on the web site but the site is working fine... in this case the severity is low but the priority is high because from company's reputation it is most imp to resolve. After all the reputation wins more clients and projects and hence increases revinue