The drops: this position gives you the most control of your bike. It’s perfect for descending. It’s closer to your brakes and you have the greatest leverage in this position. This is the safest position to ride in since you can maintain a good grip at all times. It’s the most aggressive position on the bike. With your hands low, it pulls you forward and makes you more aerodynamic.
The hoods: this is the most common position to spend the majority of the time on your bike. The hoods put your slightly more upright than the drops and should make you more comfortable, but you can also brake and change gear from this position too. It is not ideal for fast descending, since your center of gravity is not low enough. Holding the hoods are great for climbing when out of the saddle.
The tops: this is great for long steady climbs. It brings your body up and tilts your hips back, which help for powerful and efficient climbing. This is the least aerodynamic of the positions. You should use this only for climbing and slow riding since your hands are far from the brakes.