Output Formats for Numbers
ScientificForm[expr]  print all numbers in scientific notation 
EngineeringForm[expr]  print all numbers in engineering notation (exponents divisible by 3) 
AccountingForm[expr]  print all numbers in standard accounting format 
Output formats for numbers.
These numbers are given in the default output format. Large numbers are given in scientific notation.
Out[1]=  

This gives the numbers in engineering notation, with exponents arranged to be multiples of three.
Out[3]//EngineeringForm= 
 

In accounting form, negative numbers are given in parentheses, and scientific notation is never used.
Out[4]//AccountingForm= 
 

NumberForm[expr,tot]  print at most tot digits of all approximate real numbers in expr 
ScientificForm[expr,tot]  use scientific notation with at most tot digits 
EngineeringForm[expr,tot]  use engineering notation with at most tot digits 
Controlling the printed precision of real numbers.
Here is ^{9} to 30 decimal places.
Out[5]=  

Options for number formatting.
All the options in the table except the last one apply to both integers and approximate real numbers.
All the options can be used in any of the functions
NumberForm,
ScientificForm,
EngineeringForm and
AccountingForm. In fact, you can in principle reproduce the behavior of any one of these functions simply by giving appropriate option settings in one of the others. The default option settings listed in the table are those for
NumberForm.
This gives an explicit plus sign for positive numbers, and uses  in place of a decimal point.
Out[10]//NumberForm= 
 

When
Mathematica prints an approximate real number, it has to choose whether scientific notation should be used, and if so, how many digits should appear to the left of the decimal point. What
Mathematica does is first to find out what the exponent would be if scientific notation were used, and one digit were given to the left of the decimal point. Then it takes this exponent, and applies any function given as the setting for the option
ExponentFunction. This function should return the actual exponent to be used, or
Null if scientific notation should not be used.
The default is to use scientific notation for all numbers with exponents outside the range 5 to 5.
Out[11]=  

Having determined what the mantissa and exponent for a number should be, the final step is to assemble these into the object to print. The option
NumberFormat allows you to give an arbitrary function which specifies the print form for the number. The function takes as arguments three strings: the mantissa, the base, and the exponent for the number. If there is no exponent, it is given as
"".
PaddedForm[expr,tot]  print with all numbers having room for tot digits, padding with leading spaces if necessary 
PaddedForm[expr,{tot,frac}]  print with all numbers having room for tot digits, with exactly frac digits to the right of the decimal point 
NumberForm[expr,{tot,frac}]  print with all numbers having at most tot digits, exactly frac of them to the right of the decimal point 
Column[{expr_{1},expr_{2},...}]  print with the expr_{i} left aligned in a column 
Controlling the alignment of numbers in output.
Whenever you print a collection of numbers in a column or some other definite arrangement, you typically need to be able to align the numbers in a definite way. Usually you want all the numbers to be set up so that the digit corresponding to a particular power of 10 always appears at the same position within the region used to print a number.
You can change the positions of digits in the printed form of a number by "padding" it in various ways. You can pad on the right, typically adding zeros somewhere after the decimal. Or you can pad on the left, typically inserting spaces in place of leading zeros.
This prints each number with room for a total of 7 digits, and with 4 digits to the right of the decimal point.
Out[18]//PaddedForm= 
 

With the default setting for the option
NumberPadding, both
NumberForm and
PaddedForm insert trailing zeros when they pad a number on the right. You can use spaces for padding on both the left and the right by setting
NumberPadding>{" ", " "}.
BaseForm[expr,b]  print with all numbers given in base b 
Printing numbers in other bases.
"Digits in Numbers" discusses how to enter numbers in arbitrary bases, and also how to get lists of the digits in a number.