Automation for JIRA #002 - What is an issue?

Today let's move back to the beginning. A Jira foundations - issues. I will try to compress this knowledge and explain it as simple as I can.
You can browse an Atlassian documentation to find many more useful information.

Issues are the building blocks of any Jira project.
An issue could represent an epic, a story, a bug, a task, or another issue type in your project. In my work I use issues that way: An epic is „a big thing” it is like a complete main menu screen for a game. Than it can be divided it into smaller pieces – stories. For example a story can be graphics for that menu. After that tasks come into action: graphics for buttons, a logo graphics, icons, etc... If it is needed also sub-tasks can be added to this complete tree of issues.
A subtask can be created for an issue to either split the issue into smaller chunks, or to allow various aspects of an issue to be assigned to different people. If you find a subtask is holding up the resolution of an issue, you can convert the subtask to an issue, to allow it to be worked on independently. If you find an issue is really just a subtask of a bigger issue, you can also convert an issue to a subtask.

To sum this up in JIRA Core, an issue is essentially a packet of work. You can create relations between them, adding attachments, adding and editing special fields, manage them.

source: confluence.atlassian.com