MathMLForm[expr] prints as a MathML form of expr.
MathMLForm gives presentation MathML, although its output can normally be interpreted by Mathematica.
MathMLForm[expr] gives MathML for the TraditionalForm of expr.
MathMLForm[StandardForm[expr]] gives MathML for the StandardForm of expr.
MathMLForm acts as a "wrapper", which affects printing, but not evaluation.
MathMLForm gives special characters using HTML aliases.
See The Mathematica Book: Section 1.11.7 and Section 2.8.1.
See also: HTMLSave, Export, TeXForm.