Like most other information systems, JIRA organizes its data in a hierarchical structure. At the lowest level, we have field, which are used to hold raw information. Then the next level up, we have issue, which are like a unit of work to be performed.
An issue will belong to a project, which defines the context of the issue. Finally, we have project category, which logically group similar projects together. We will discuss each of these levels in the forthcoming sections. The figure in the following illustrates the hierarchy we just talked about:
Project category is a logical grouping for projects, usually of similar nature. Project category is optional. Projects do not have to belong to any category in JIRA. When a project does not belong to any category, it is considered uncategorized. The categories themselves do not contain any information; they serve as a way to organize all your projects in JIRA, especially when you have many of them.