Number

This gives you control over the number of decimal places displayed as well as the option of displaying basic punctuation, comma's and parentheses.
The default is 2 decimal places and negative numbers are displayed in black preceded by a minus sign.

Decimal places - The number of decimal places can be from 0 to 30 and this can be changed by either typing directly into the box or using the spinner button.
Use 1000 separator - The 1000 separator will insert a comma every 3 digits, i.e. hundred, thousand etc.
Negative Numbers - The default is to display negative numbers with a minus sign, although there are the options to make negative numbers red, or display them in red with a negative sign.

How it is different to the General format

This number format is like the General format but with three differences:
You can define the number of decimal places.
Allows you to use a comma seperator between groups of three digits (handy when working with large numbers).
Negative numbers can be displayed in red and or with a negative sign.

Home Tab

You can quickly add the 1000 separator to your cells by pressing the following button on the Home tab or the Mini toolbar.

Comma Style - Adds a comma and two decimal places to the number by applying the format "#,##0.00".

All Numbers in Black

This is the default

Negative Numbers in Red

No minus sign is displayed because negative numbers are displayed in red

Negative Numbers in Parentheses

Negative Numbers in Red with Parentheses

Excel stores numbers as numeric data even if you later apply the text format to cells that contain numbers.