$MaxNumber gives the maximum arbitrary-precision number that can be represented on a particular computer system.

$Path gives the default list of directories to search in attempting to find an external file.

$Pre is a global variable whose value, if set, is applied to every input expression.

$PrePrint is a global variable whose value, if set, is applied to every expression before it is printed.

$PreRead is a global variable whose value, if set, is applied to the text or box form of every input expression before it is fed to Mathematica.

$TemporaryDirectory gives the main system directory for temporary files on your computer system.

The way modules work in Mathematica is basically very simple. Every time any module is used, a new symbol is created to represent each of its local variables. The new symbol ...

LayeredGraphPlot attempts to draw the vertices of a graph in a series of layers, placing dominant vertices at the top, and vertices lower in the hierarchy progressively ...

Numerical integration functions. This finds a numerical approximation to the integral ∫_(0)^∞ e^-x^3 x. Here is the numerical value of the double integral ∫_(-1)^1 dx ...

Testing for numeric quantities. Pi is a symbol, so Pi+3 is not explicitly a number. It does however have a numerical value.