Difference between post-dated check and stale check?

A Post Dated Cheque is one that has a date in future. A Stale Cheque is one in which the date is in the Past. Usually cheques have a validity of around 90 to 120 days.

So, lets say someone gave you a cheque in March 2011 and you have still not cashed it, it is a stale dated cheque.

Similarly if I give you a cheque with date as 10-May-2012 today (on 14 Jan 2012) it would be a Post dated cheque. In this case, the cheque is valid only on or after 10th May 2012. Until then, it is just a piece of paper and is worthless