FractionBox[x, y] represents in input and output.
Inside \( ... \) FractionBox[x, y] can be input as x \/ y.
In a notebook a FractionBox can be created using /. moves out of the fraction.
In StandardForm and InputForm, FractionBox[x, y] is interpreted on input as x/y.
The axis of FractionBox[x, y] is taken to go through the fraction line. The baseline lies below the axis by the distance between the axis and the bottom of characters such as ( in the current font.
The width of the fraction line can be given in x-heights as the setting for the SpanLineThickness option in StyleBox.
If FractionBox[x, y] does not fit on a single line, it is output as x / y.
In StandardForm, explicit FractionBox objects are output literally. You can use DisplayForm to see the display form of such objects.
See The Mathematica Book: Section 2.8.10.
See also: OverscriptBox, GridBox.