Documentation for JIRA 5.2. Documentation for other versions of JIRA is available too.

The keyboard shortcuts dialog box shows an overview of JIRA actions that are available as keyboard shortcuts and the combination of keystrokes required to perform them.

Choose your username's dropdown, then choose Keyboard Shortcuts.
You can also open this dialog box by pressing '?' ('Shift + /') on your keyboard. Note, your cursor must not be focused in a JIRA text field for '?' to work.