Styles are pre-defined combinations of various formatting options that make it easier to keep the presentation of the text uniform. What is more, since a style often contains a number of features at once, when you want to customize the way a text fragment looks, you do not need to change a font, its size, or text and background color separately.
To choose a style, select a text fragment and press the toolbar button. The Styles drop-down list contains a number of pre-defined block and inline styles that you can use. To make the choice easier, the style names are displayed in a style that they represent, giving you a preview of what the text will look like.
A style can be applied to the whole document, a paragraph, or a text fragment of arbitrary length — even a single letter. Do note, however, that for esthetic and readability reasons it is recommended not to combine too many different styles in one document and to keep the styling consistent.