$InstallationDirectory (Built-in Mathematica Symbol)

$InstallationDirectory gives the top-level directory in which your Mathematica installation resides.

$Linked (Built-in Mathematica Symbol)

$Linked is True if the Mathematica kernel is being run through MathLink.

$MachineID (Built-in Mathematica Symbol)

$MachineID is a string that gives, if possible, a unique identification code for the computer on which Mathematica is being run.

$ProcessorType (Built-in Mathematica Symbol)

$ProcessorType is a string giving the architecture of the processor on which Mathematica is being run.

$ReleaseNumber (Built-in Mathematica Symbol)

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

$System (Built-in Mathematica Symbol)

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

$SystemWordLength (Built-in Mathematica Symbol)

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

$UserDocumentsDirectory (Built-in Mathematica Symbol)

$UserDocumentsDirectory gives your default "documents" directory.

$Version (Built-in Mathematica Symbol)

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

Data Formats in Wolfram|Alpha (Mathematica Overview)

Introduction Exploring Data Formats Interactively Obtaining Data Formats Programmatically