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.
Flat top pistons are very similar in nature to stock pistons, running at medium compression ratios. Consequently, they work great in many applications and performance levels. Valve reliefs are designed with appropriate clearance to fit most types of standard and aftermarket heads.