As an open data portal grows beyond 25-30 datasets, it is more helpful to visitors if they can browse for data by a subject matter or theme. These categories are short, at most a word or two, and allow related data to be grouped together. Categories also empower visitors to explore available data for inspiration, rather than requiring them to use a search tool to find something specific.
Creating categories is often a fundamental step when implementing an open data portal. Categories do not need to be permanent; it makes sense to have three to four categories for a small number of datasets, and re-evaluate them on an annual basis as more data is published. Most mature open data portals have 8 to 12 categories. Having too many might mean that the categories are not broad enough. Having too few, especially when combined with a large number of datasets, might mean that the categories are too broad and less helpful for visitors.
Although there is no consistent set of categories between open data portals, the following are quite common and might serve as a starting point: Business, Education, Environment, Finance, Health, Human (or Social) Services, Property, Public Safety, Recreation, and Transportation. A librarian or information architect can provide insight and assistance when creating or revising the list of categories.