In AEM RichText is a very important component. This component can be used seperately or this can be be embedded in other components. This can also be called as WYSIWYG (What you see is what you get). In Rich text we can perform below activities. Adding hyperlinks. Adding Bold,Italic,underlined Adding bullets/numbers. Adding Headings. Adding specific colors to text. Adding special characters. Adding superscript/subscript. Table can be added. Adding custom Html/css with in the edit source icon. Having good knowledge on HTML/CSS will help in customising the Rich Text. For more information on AEM , go through the below syllabus. AEM Author Concepts
AEM Author Tutorial, which will give overview of content authoring. This helps from beginners to professionals