Curvature is a texture that effectively samples the underlying mesh for curves. A region around each shaded point gets sampled with additional rays (similar to Dirt) and all additional normals are averaged to get a "smooth" normal at the original point. This normal is used to compute the color. The color at each shaded point is a greyscale value: darker values are dents (or holes), lighter values are peaks. The parameters should be adjusted by trail and error since they generally depend on the scale used, hence the defaults might not be appropriate at all in some cases.

Sample Spread – Specifies the radius of the sampled region. If a texture is selected, it overrides the color as long as the texture checkbox is enabled.

Min Output Color – Specifies the minimum output color for the texture.

Max Output Color – Specifies the maximum output color for the texture.

Scale – Adjusts the scale relative to which the curvature is measured. 

Threshold – Curvature values below this threshold are ignored and are shown as pure black. This focuses the curvature effect only around the most curved areas of the mesh.

Mode – Specifies the multiplication mode of the colors. 

Multiply – The color used for blending is black.
Blend Amount –  The color used for blending is the one specified in the color slot.

Sample Spread – Blends between a color and a texture, if specified.