OutputForm[expr] prints as a two-dimensional representation of expr using only keyboard characters.
OutputForm is an approximation to StandardForm which uses only ordinary keyboard characters.
The OutputForm of many kinds of expressions is quite different from their internal representation.
OutputForm acts as a "wrapper", which affects printing, but not evaluation.
OutputForm cannot be used directly for input to Mathematica.
When possible, OutputForm uses approximations to special characters. Thus is given as >= and as e'.
See The Mathematica Book: Section 1.10.9 and Section 2.8.1.
See also: StandardForm, TraditionalForm, InputForm, TeXForm, MathMLForm, Short, FullForm.