Excel has a few different data types for each cell and each one will have a slightly different formatting or numerical effect. To select a data type highlight a cell/cells and click the drop down called "General".

From here you can select a data type. The available data types are:

General
The default number format that Excel applies when you type a number.
Number
Used for the general display of numbers. You can specify the number of decimal places that you want to use, whether you want to use a thousands separator, and how you want to display negative numbers.
Currency
Used for general monetary values and displays the default currency symbol with numbers. You can specify the number of decimal places that you want to use, whether you want to use a thousands separator, and how you want to display negative numbers.
Accounting
Also used for monetary values, but it aligns the currency symbols and decimal points of numbers in a column.
Date
Displays date and time serial numbers as date values, according to the type and locale (location) that you specify.
Time
Displays date and time serial numbers as time values, according to the type and locale (location) that you specify.
Percentage
Multiplies the cell value by 100 and displays the result with a percent (%) symbol. You can specify the number of decimal places that you want to use.
Fraction
Displays a number as a fraction, according to the type of fraction that you specify.
Scientific
Displays a number in exponential notation, replacing part of the number with E+n, where E (which stands for Exponent) multiplies the preceding number by 10 to the nth power.
Text
Treats the content of a cell as text and displays the content exactly as you type it, even when you type numbers.
Special
Displays a number as a postcode, phone number etc.
Custom
Allows you to modify a copy of an existing number format code. Use this format to create a custom number format that is added to the list of number format codes.