This blog has a definition of epics in JIRA:
Epics are significantly larger bodies of work. Epics are feature-level work that encompasses many user storie
Addition: Atlasian now have created a new article explaining this from their perspective.
https://www.atlassian.com/agile/delivery-vehicles
My opinion / usage.
Labels and Components are almost straightforward and well answered already.
Components examples
Labels examples.
But let me give my two cents about Epics because i find this phrase way too generic.
Epics are significantly larger bodies of work
Larger? 10 Sprints? 10 Stories? 20 Stories? or what?
Personally i would classify Epics as Goals.
At a Yearly / Quarter Retrospective your company holds a meeting with all members and stakeholders , and concludes to the following
This would mean 3 epics with set of stories to cover each of those generic requirements