An issue are items that are used to track individual pieces of work that must be completed. Jira uses issue to track the tasks. They also helps to track all work that underlie in project.
There are 5 issues we can create:
a) Story: Story is user’s requirement, A story is a piece of work that can be completed within a sprint.
b) Task: A task has to be completed to achieve teams goal.
c) Subtask: It is subtask of issues. in a logged issue there can be different ways to resolve it. this is called Subtask.
d) Bug: A problem that impairs or prevents the function of the product
e) Test: A Test issue is used to write test condition.