InputForm[expr] prints as a version of expr suitable for input to Mathematica.
Example: InputForm[x^2 + 1/a] .
InputForm always produces one-dimensional output, suitable to be typed as lines of Mathematica input.
InputForm acts as a "wrapper", which affects printing, but not evaluation.
Put (>>) produces InputForm by default.
Short[InputForm[expr]] can be used, but may generate skeleton objects which cannot be given as Mathematica input.
The option NumberMarks can be used to specify whether ` marks should be used to indicate type, precision or accuracy of approximate numbers.
See The Mathematica Book: Section 1.9.4, Section 1.10.9 and Section 2.8.1.
See also: OutputForm, FullForm, StandardForm.