Hi Dave,
no Problem. :-)
Categories can be used to collect issues of the same kind (tracker) under a specific topic. For example I have "Infrastructure", "Services", "Support", "Documentation" as categories for my issues. Just to get a better overview for what the most work is done. This could be realised with custom fields, too but the advantage of the categories is that they are project specific. So every project can have its own list of categories. One project can have different servers as categories, another project likes to categorize the tasks for different facilities, you name it.
You can group your issues by category in the issues list, generate reports about spent time by category or show a graph of open and closed issues by category in the roadmap.
Greetings,
Bernhard