ButtonStyle is an option for ButtonBox which specifies the default properties for the button.
Typical styles defined in the standard notebook front end are:
The properties specified by a button style can affect both the appearance and action of a button.
The properties can be overridden by explicit settings for ButtonBox options.
See The Mathematica Book: Section 2.8.11 and Section 2.10.6.
See also: ButtonFrame, ButtonFunction.