Does percent and percentage different from each other? If yes, WHY? If no, WHY?

Percent and percentage are closely related. The difference is in their use:

Use percent when you know or are looking for or using a specific number. For example the solution is 15% chlorine, the bank charges 15% on signature loans, the population is increasing 2% per year.

Use percentage without a specific number. If percentage is used as a noun, there needs to be an adjective. For example, a large percentage of the population has no land line, the percentage of people who smoke is declining.

You would not ask for the percentage of land devoted to farming; you would ask for the percent. On the other hand, you might speak of a large percentage of land devoted to corn. The difference is solely in the usage.