Issue types

1. Epics are large user stories which do not fit in a single sprint.
2. Stories are tasks in the given/when/then format. Useful for collecting a bunch of related subtasks.
3. Spikes are research tasks with a documented outcome which help in deciding what tickets to create to meet one or more goals.
4. Discovery Epics are similar to spikes but substantially larger.

See https://www.atlassian.com/agile/project-management/epics-stories-themes for more details.