Answer:
The two terms are linked concepts, in that horizontal policy approaches are used to address cross-cutting issues. When an issue is "cross-cutting", it suggest the image of a public policy issue that cuts across the responsibilities of a number of government departments. In order to address such an issue, governments attempt to take a horizontal policy approach to solving them, in which responsibility for addressing the issue is spread across a number of departments. Contrasted to this, an issue which can be effectively handled by a single Ministry or department would be amenable to a vertical policy approach.
An example would be efforts to address climate change. This may, on first consideration, seem like a matter for a department of environment. So departments responsible for energy, transportation, natural resources, agriculture, industry and a range of other narrow policy issues must be brought together to work towards effective policy responses.