Integer is the head used for integers.
_Integer can be used to stand for an integer in a pattern.
Integers can be of any length.
You can enter an integer in base b using b^^digits. The base must be less than 36. The letters are used in sequence to stand for digits 10 through 35.
See The Mathematica Book: Section 3.1.1.
See also: IntegerDigits, BaseForm, IntegerQ, Integers.