$ReleaseNumber is an integer which gives the current Mathematica kernel release number, and increases in successive releases.

$System is a string describing the type of computer system on which Mathematica is being run.

$SystemWordLength gives the effective number of bits in raw machine words on the computer system where Mathematica is running.

$UserDocumentsDirectory gives your default "documents" directory.

$Version is a string that gives the version of the Mathematica kernel you are running.

Introduction Exploring Data Formats Interactively Obtaining Data Formats Programmatically

Reading and Writing Mathematica Files External Programs Streams and Low-Level Input and Output

Importing and Exporting Data Importing and Exporting Files Exporting Graphics and Sounds

Entering Greek Letters Entering Two-Dimensional Input Editing and Evaluating Two-Dimensional Expressions

Constructing Matrices Getting and Setting Pieces of Matrices Scalars, Vectors, and Matrices