A Dish Top style piston will effectively lower your engine's compression ratio by removing material from the top of the piston in a circular, "dish" shape. They are primarily used in boosted (supercharged, turbocharged) applications where low to medium compression ratios are desired.
A Solid Dome top piston will effectively raise an engine's compression ratio, making them the ideal type of piston for medium to higher performance engines that may be using power adders (such as nitrous). Dome Top pistons range from medium to high compression ratios.