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Xcode is a suite of tools for developing software on Mac OS X, developed by Apple. Xcode 3.2, the latest major version, is bundled free with Mac OS X v10.6, though it is not installed by default. Because version 3.2 is not supported on older Mac OS versions, more dated versions of Xcode are free from the Apple Developer Connection.
The main application of the suite is the integrated development environment (IDE), also named Xcode. The Xcode suite also includes most of Apple's developer documentation, and Interface Builder, an application used to construct graphical interfaces.

The Xcode suite includes a modified version of free software GNU Compiler Collection (GCC, apple-darwin9-gcc-4.2.1 as well as apple-darwin9-gcc-4.0.1, with the former being the default), and supports C, C++, Fortran, Objective-C, Objective-C++, Java, AppleScript, Python and Ruby source code with a variety of programming models, including but not limited to Cocoa, Carbon, and Java. Third parties have added support for GNU Pascal,[1] Free Pascal,[2] Ada,[3] C#,[4] Perl,[5] Haskell,[6] and D. The Xcode suite uses GDB as the back-end for its debugger.

Source:
1. Wikipedia-Xcode
2. Apple Corp.