In TextEdit, you can create documents in plain text or Rich Text Format (RTF). Plain text is very simple and does not allow any formatting. Rich text allows you to add images and other media files, and to use special formatting such as bold or italic text. You can also save RTF documents in a variety of other file formats, such as HTML or Web Archive.
You can change the format of a single document while you’re working in it, and you can specify the default format for all new TextEdit documents.
To change the format of a document:
To make the file plain text, choose Format > Make Plain Text.
To make the file rich text, choose Format > Make Rich Text.
To set the default format for all new documents:
Choose TextEdit > Preferences, and then click New Document.
Select “Rich text” or “Plain text” below Format.