Let's take a look at the architecture of JIRA; it will help you to understand the core concepts:
Project Categories: When there are too many projects in JIRA, it becomes important to segregate them into various categories. JIRA will let you create several categories that could represent the business units, clients, or teams in your company.
Projects: A JIRA project is a collection of issues. Your team can use a JIRA project to coordinate the development of a product, track a project, manage a help desk, and so on, depending on your requirements.
Components: Components are subsections of a project. They are used to group issues within a project to smaller parts.
Versions: Versions are a point-in-time for a project. They help you schedule and organize your releases.
Issue Types: JIRA will let you create several issue types that are different from each other in terms of what kind of information they store. JIRA comes with default issue types, such as bug, task, and subtask, but you can create more issue types that can follow their own workflow as well as have different sets of fields.
Sub-Tasks: Issue types are of two types—standard and subtasks, which are children of a standard task. For instance, you can have test campaign as a standard issue type and test cases as subtasks.