You can check the Text Formatting Notation (http://jira.atlassian.com/secure/WikiRendererHelpAction.jspa?section=all) that contains all you can do in JIRA.
For more details, I'd also recommend you to check the Editing Rich Text Fields (https://confluence.atlassian.com/display/JIRA/Editing+Rich-Text+Fields) documentation.
Apart from that, you can use add-ons like the JEditor (https://marketplace.atlassian.com/plugins/com.jiraeditor.jeditor) that will enable a more feature-rich text editor.
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