This is documentation for Mathematica 8, which was
based on an earlier version of the Wolfram Language.
 MATHEMATICA TUTORIAL

Specifying Types of Expression in Patterns

You can tell a lot about what "type" of expression something is by looking at its head. Thus, for example, an integer has head Integer, while a list has head List.
In a pattern, and represent expressions that are constrained to have head h. Thus, for example, represents any integer, while represents any list.
 x_h an expression with head h x_Integer an integer x_Real an approximate real number x_Complex a complex number x_List a list x_Symbol a symbol

Patterns for objects with specified heads.

This replaces just those elements that are integers.
 Out[1]=
You can think of making an assignment for as like defining a function that must take an argument of "type" Integer.
This defines a value for the function when its argument is an integer.
The definition applies only when the argument of is an integer.
 Out[3]=
The object has head Real, so the definition does not apply.
 Out[4]=
This defines values for expressions with integer exponents.
The definition is used only when the exponent is an integer.
 Out[6]=