Percentages in Excel are stored as decimal values. For example, 25% is stored as the value 0.25, 50% is stored as the value 0.5, etc.
It is only the formatting of a cell that makes the underlying decimal value appear as a percentage.
To change a cell's formatting to display percentages:
The result of formatting (with zero decimal places) is shown below.
Values Displayed as Decimals:

After Formatting as a Percentage:

When calculating percentages in Excel, there are two types that are typically used. These are:
Percentage as a Proportion 
Eg: If, out of 120 questions, you answer 90 correctly, how is this expressed as a percentage score? 
Percentage Change 
Eg: If your sales figures are $1,250 in January and $1,750 in February, what is the percentage increase in February, compared to January? 
These two percentage calculations are discussed separately below.
Formulas:

Results:

If you want to calculate a sample of a full set, as a percentage, this is done by dividing the sample size by the size of the full set.
For example, if you take an exam and answer 90 out of 120 questions correctly, this can be expressed as a percentage by simply dividing 90 by 120. This gives the result 0.75, or 75%
This percentage calculation is shown in the Excel spreadsheet above.
Note that, to get the result shown in this example, it is necessary to set cell B3 to have percentage formatting.
If you want to calculate the percentage change between value a and value b, this is done using the following formula:
Formulas:

Results:

For example if your sales figures are $1,250 in January and $1,750 in February, the percentage change in February, compared to January, is given by:
This gives the result 0.4, or +40%
This percentage calculation is shown in the Excel spreadsheet above.
As in the previous example, in order to get the result displayed as a percentage, it is necessary to set cell B3 to have percentage formatting.
Further examples of Calculating Percentages in Excel can be found on the Microsoft Office website.