ScientificForm[expr] prints with all real numbers in expr given in scientific notation.
ScientificForm[expr, n] prints with numbers given to n-digit precision.
ScientificForm takes the same options as NumberForm, but uses a different default function for ExponentFunction.
You can mix ScientificForm and BaseForm.
ScientificForm acts as a "wrapper", which affects printing, but not evaluation.
See The Mathematica Book: Section 2.8.7.
See also: EngineeringForm, NumberForm.