From Wikipedia:

Cross-stitch is a form of sewing and a popular form of counted-thread embroidery in which X-shaped stitches in a tiled, raster-like pattern are used to form a picture. Cross-stitch is often executed on easily countable fabric called aida cloth, whose weave creates a plainly visible grid of squares with holes for the needle at each corner. Counted cross-stitch projects are worked from a gridded pattern called a chart and can be used on any count fabric; the count of the fabric and the number of threads per stitch determine the size of the finished stitching.