As your vehicle interior ages, exposure to the sun's ultra violet waves and extremes in temperature dries out the vinyl surfaces of the interior. Because the dash faces skyward, is located right under the windshield and stretched tight over the dash, it tends to dry out quicker than other vinyl surfaces. When this happens, unsightly crack and breaks will appear in the dash.
If the cracks are small, you may be able to repair them using a vinyl repair kit available from any auto parts store. These kits cost only a few dollars, come with easy-to-follow instructions and have a variety of vinyl colors so you can color match your dash.
You can also purchase a carpet cover that lays over the dash, covering the cracks. These covers are available at auto parts and discount stores for just about every make and model, cost between $30 and $90 and install in minutes. They have the added advantage of eliminating most of the dashboard reflections in the windshield. Dash covers will keep an uncracked dash from cracking if they are installed when the dash is new and hasn't dried out. If you do this, it is wise to remove the cover every once in awhile and treat the dash with a vinyl protectant that keeps it from drying out.
If you want to keep an original appearance, or don't like carpeted dashboards, you can opt for a dash panel overlay. These overlays are made from molded plastic and fit over a damaged dash to make it look like new. They should be installed by a body and trim shop, and while somewhat costly, they are an inexpensive alternative to replacing an entire dashboard.