Integers represents the domain of integers, as in x Integers.
x Integers evaluates immediately only if x is a numeric quantity.
Simplify[expr Integers] can be used to try to determine whether an expression is an integer.
IntegerQ[expr] tests only whether expr is manifestly an integer (i.e., has head Integer).
Integers is output in TraditionalForm as .
See The Mathematica Book: Section 1.4.6 and Section 3.3.10.
See also: Element, Simplify, IntegerQ, Reals, Primes, Algebraics.