In general, it's best to just give dates, and say nothing specific.
However, it's now getting increasingly common for new employers to ask specific questions. Certainly, when I applied for my last job, my new employer sent a big questionnaire to my referee asking stuff like:
What dates did X work for you?
What position did he hold?
What skills were required for this position?
How many days off sick did he have in the last 2 years?
Has X ever been called before a disciplinary panel? (yes/no)
Have any co-worker ever raised concerns about X's behavior or conduct (including abuse of alcohol or other substances)? (yes/no)
Has X complied with and completed all appropriate training given as part of the job? Were there any concerns over progress? (yes/no)
etc.
There was a warning, stating that if the questionnaire was not completed in full, it would be regarded as invalid, and would not be passed to the HR panel, and as a result employment could not be considered.
If you get a request for a reference that looks like that - then you have to answer each question truthfully. If the new employer subsequently hires them, on the basis of your reference, but later finds out that it was incorrect (e.g. a co-worker had made a complaint to his manager that X had turned up to work drunk), then you can be sued.