Arena is a complete GUI for chess engines. The program already comes with an engine that enables you to play against the computer, but it also supports almost all popular engines. The program is vastly customizable and easy to use, so you create exactly the kind of game you want. Thanks to the adjustable strength, you can select the difficulty level and increase it as you get more acquainted with game and more skillful.
What is great is that the program provides detailed information about moves and thinking process of the computer. This can help you learn from the engine to defeat your enemies faster. You can even play online with other people or create tournaments between two engines to learn from them too. Other useful features of the program include printing capabilities, support of the DGT Chessboard, EPD and PGN support, user interface available in multiple languages, to name but a few.
In short, Arena is a wonderful program that includes everything chess players need to become professional players.