From LQWiki

Swing is a GUI toolkit for Java. It extends and largely replaces AWT. The main difference between Swing and AWT is that the Swing GUI components are written in 100% Java code (and as such do not use the platform's native GUI widgets). This allows for pluggable Look-and-Feel/Themes. Swing replaces the "old" AWT widgets and also adds many modern ones of its own. The names of the Swing classes all start with a 'J', for example AWT's Label class becomes JLabel with Swing.

It's in the standard Java class library since Java 2 (version 1.2) (though available as a separate library for 1.1)

See also

• SWT - An alternative Java GUI toolkit.