EngineeringForm[expr] prints with all real numbers in expr given in engineering notation.
EngineeringForm[expr, n] prints with numbers given to n-digit precision.
In "engineering notation" the exponent is always arranged to be a multiple of 3.
EngineeringForm takes the same options as NumberForm, but uses a different default function for ExponentFunction.
You can mix EngineeringForm and BaseForm.
EngineeringForm acts as a "wrapper", which affects printing, but not evaluation.
See The Mathematica Book: Section 2.8.7.
See also: ScientificForm, NumberForm.