Semivariance is the squared difference between two points divided by two. A variogram cloud plots all of the squared differences of all pairs of points as a function of separation distance. A variogram is the expected squared difference of all pairs of points for a separation distance.
The nugget is the value of the semivariance at the starting location, which reflects the measurement error. The sill is where the semivariogram reaches its maximum height. The range is the distance beyond which the variogram value doesn't change much. A partial sill is defined as the difference between the sill and the nugget.
There are different variogram models, including pure nugget effects, linear models, spherical models, Gaussian models, and ...
