Christine Mann has posted a Guide to Needlepoint Canvas over at Suite101. The article summarizes the differences between the various types of needlepoint canvas and is a pretty good introduction to this topic.
Good quality materials are essential for creating works of art that last, and they help avoid frustration while you work. Before beginning a needlepoint project, inspect the canvas to make sure the threads have no knots or cuts. The grid should be even, not warped or distorted. Make sure the weave and gauge of the canvas is the right scale for the design and yarn you plan to use. Make sure its stiffness and sturdiness are a good match for the way the needlepointed item will be used.
In other words, it pays to buy designs that were done on quality canvas. At Pepita Needlepoint, all designs are printed on expensive Zweigart Mono Deluxe canvas, widely preferred by stitchers everywhere. Threads are clean and straight, the weave is uniform and stiff, and the material is sturdy yet gentle on your hands.