Surface roughness:

Surface roughness often shortened to roughness, is a component of surface texture. It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. Roughness is typically considered to be the high-frequency, short-wavelength component of a measured surface. However, in practice it is often necessary to know both the amplitude and frequency to ensure that a surface is fit for a purpose. [Wikipedia]

Ra: Is the mean arithmetical value in μm is assumed for the roughness measurement.
Rz: Is the average roughness in μm measured in 10 points.
Rt: Maximum value of roughness or height from the lowest point to the highest point.

http://www.lucefin.com/wp-content/files_mf/roughness.pdf