A Style is simply a Condition saved to the database for use in other bids - sort of like a template. To learn more about creating Conditions and Saving Conditions as a Style, refer to - Conditions. Styles, Style Sets and Zones.
The Condition Style Library contains all the saved Styles for your database. A Condition Style may be modified at any time. Once a condition is saved as a Style it automatically becomes part of the Style Library.
See Creating a Style for more information.