Twitter-Bootstrap buttons are awesomely beautiful. Try them out by scrolling over them
In Twitter Bootstrap bootstrap 3.0.0, Twitter button is flat. You can customize it from http://getbootstrap.com/customize. Button color, border radious etc.
Also you can find the HTML code and others functionality http://twitterbootstrap.org/bootstrap-css-buttons.
Bootstrap 2.3.2 button is gradient but 3.0.0 ( new release ) flat and looks more cool.
and also you can find to customize the entire bootstrap looks and style form this resources: http://twitterbootstrap.org/top-5-customizing-bootstrap-resources/