Text and elements within the article content can be formatted and styled using various buttons in the editor toolbar and in some cases using shortcut keys, like CTRL+B (Bold).
By Formatting or Styling we are referring to an action that changes the appearance of the text or element. This is done by applying a style, assigning a class or by changing the element type. For example, making text Bold using the
button (or CTRL+B keyboard shortcut) places the text within a <strong> element ie: text, while Underlining some text using the 
button (or CTRL+U keyboard shortcut), adds the css style text-decoration:underline; to the text, ie: <span style="text-decoration:underline">text</span>. Of course as this is a WYSIWYG editor you don't see the code just the formatted text.
Before any text or element van be formatted it must first be selected. This is usually done by clicking on the element, clicking and dragging to select blocks of text or double-clicking on single word to select it. See Selecting text and elements
Applying a class from the Styles list
The Styles list is a list of classes used by your sites template. To apply a class, select the text or element and select the class from the Styles list. The class will be applied to the element using the class HTML attribute, eg: class="some_class". In the case of a text selection the class will be added using a span element, eg: <span class="come_class">selected text</span>
Applying or editing a style using the Edit CSS Style button
Existing styles added using any of the toolbar buttons can be edited, or new styles added to text or an element, using the Edit CSS Style 
dialog.
Removing a style
Formatting can be a removed using the Remove Format 
button or by using the Edit CSS Style dialog 