FullForm[expr] prints as the full form of expr, with no special syntax.
Example: FullForm[a + b^2] .
FullForm acts as a "wrapper", which affects printing, but not evaluation.
See The Mathematica Book: Section 2.1.1, Section 2.1.1, Section 2.1.4, Section 2.3.1, Section 2.3.13 and Section 2.8.1.
See also: InputForm, TreeForm.