If you choose <em>Text (plain)</em> or <em>Text (plain, long)</em> you restrict the input to <em>Plain text</em> only. If you choose <em>Text (formatted)</em>, <em>Text (formatted, long)</em>, or <em>Text (formatted, long with summary)</em> you allow users to write formatted text. Which options are available to individual users depends on the settings on the <a href=":formats">Text formats and editors page</a>.