Generally speaking, a style in Mail & Deploy is a collection of properties and settings that dictate how an item (e.g. text, a table cell etc.) is rendered in a report document. These style settings allow you to manage styles (see Styles) which can be used to control the visual representation of report elements (see Report Elements).
You can manage styles by clicking Tools > Settings > Manage Styles and then using the following buttons:
Each style has different attributes and usages (see Styles).