An employment verification is a response to an inquiry by a prospective employer, a government agency, or an outside entity, such as a lending institution, that the current or former employee is or was employed by your organization. In most cases, the requesting organization wants verified: dates of employment, salary information, and whether the individual is still employed by your organization.
In the case of employment verification requests by prospective employers, information about the employee's performance and potential rehire are often requested. It is also not unusual for the employment verification to request job or title and salary history, in addition to current / most recent employment information.
It is up to the employer how much information to release, but an employment verification policy, that is consistently enforced, should exist.