The gray level co-occurrence matrix (GLCM) is a statistical method of texture analysis that describes texture through a set of statistical measures extracted from a matrix that computes how often pairs of pixels with specific values and in a specified spatial relationship occur in an image.
The GLCM is a tabulation of how often different combinations of pixel brightness values (gray levels) occur in an image for a certain pixel offset (distance and direction). This approach consists in representing texture through a feature vector whose components are statistics computed from the different GLCMs corresponding to the considered distance-direction combinations.
A good start on the field is having a look at The GLCM Tutorial.
