A heuristic consists of preferences that help you decide in a situation where you do not have enough information or do not care enough to make an informed decision. For example, when you want to buy yoghurt, but are no nutritionist, you might decide on which yoghurt you buy by the familiarity of the brand name (you prefer the familiar, this is called the familiarity heuristic) and other aspects that have nothing to do with the yoghurt itself.
A schema is a workflow or storyboard that tells you what to do in a recurring situation. For example, when you go shopping you might follow an internal script to make shopping more efficient: you pick the same products in the same sequence every time. You know in advance what you will buy, there are no decisions involved, in fact you ignore all alternative products. A schema allows you to navigate familiar, recurrent and similar situations by following the same sequence of actions.
Schema and Heuristic are two different things- not really related to one another, so I'm not sure asking for the difference between the two makes sense. That said-
A Schema is a cognitive framework for storing information and relating it together. Schemas form the basis for knowledge in the head.
A Heuristic is a mechanism for solving problems.
Therefore, a schema is more associated with what topics a person might make decisions upon, whereas a heuristic is more associated with how a person makes those decisions. Heuristic methods very well may depend upon existing schema; however, schema do not necessarily require the existence of a heuristic method.