Created by Frank Drake in 1961, the original Drake equation was designed to calculate the number of intelligent communicative species which could be expected to exist in the galaxy. To accomplish this the equation multiplied a series of terms such as the rate of star formation, the fraction of stars with planetary systems, the number of planets in each system, etc. The number of intelligent species was simply the product of all these terms. Many different versions have come to exist over the centuries since the original was created; the original equation is reproduced here.
When the original equation was created only a few of the terms were known with any significant degree of accuracy; the real value of the equation, then, lay in the debate it promoted rather than the answers it produced. As Humanity has expended into space it has become possible to gain a greater understanding of astrophysics and a sampling of the distribution of life in a statistically significant percentage of the galaxy, allowing the equation to be used with a much higher level of confidence.