Principal:

The money borrowed or lent out for a certain period is called the principal or the sum.

Interest:

Extra money paid for using other's money is called interest.

Simple Interest (S.I.):

If the interest on a sum borrowed for certain period is reckoned uniformly, then it is called simple interest.
Let Principal = P, Rate = R% per annum (p.a.) and Time = T years.
Then,
Simple Interest (SI) = P x R x T/100

Compound Interest (C.I.)

Let Principal = P, Rate = R% per annum, Time = n years

When interest is compound Annually:
Amount = P(1+R/100)ⁿ

When interest is compounded Half-yearly:
Amount = P[1+(R/2)/100]²ⁿ

When interest is compounded Quarterly:
Amount = P[1+(R/4)/100]⁴ⁿ