AccountingForm[expr] prints with all numbers in expr given in standard accounting notation.
AccountingForm[expr, n] prints with numbers given to n-digit precision.
AccountingForm never uses scientific notation.
AccountingForm uses parentheses to indicate negative numbers.
AccountingForm takes the same options as NumberForm, but uses a different default function for ExponentFunction, and a different default for NumberSigns.
AccountingForm acts as a "wrapper", which affects printing, but not evaluation.
See The Mathematica Book: Section 2.8.7.
See also: PaddedForm, NumberForm.