http://mathworld.wolfram.com/Point-LineDistance2-Dimensional.html
Im trying to understand the two dimensional line distance formula from the above Link but dont really understand a part of it. The divisor is the pythagoras distance formula I believe but what is the dividend? The part above the division? Not really sure.
The authors of that page computed the projection of $(x_0,y_0)$ on the line $ax+by+c=0$. Then they computed the distance from $(x_0,y_0)$ to that projection. The final result has some resemblance with the cross-product of two vectors because the cross-product of two vectors is orthogonal to each of them.