From Halopedia, the Halo wiki
Introduced in Halo 2, Drones are flying enemies that attack the player in swarms. While individually weak and lighly armed, they make up for it with agility and numbers.
Physically, Drones are weak, even more so than Major Grunts but about as resistant as Minor Grunts. However, they are rarely encountered alone, often being in large swarms, but sometimes providing support for other foes. Most of the time, Drones wield lightweight weapons such as the Needler and Plasma Pistol, though they never overcharge their Plasma Pistol to break a shield. When encountered in swarms, Drones prefer to stay at a distance and use long-range tactics, moving constantly to throw off their enemy's aim while raining a hail of plasma and needles. When in support of other Covenant, they use their aerial mobility to flank the enemy and force them out of cover, in the line of sight of their heavier armed allies. However, they cannot fly forever, and will eventually land on a wall or the ground, where they will be more vulnerable.
Drones are also capable of boarding vehicles. While they cannot drive, Drones will claw relentlessly at the pilot until he is killed or forced out. However, their melee is weak, and a player can resist out until someone shoots the drone down. Alternatively, you can crash into a structure, causing the drone to lose its grip on you.
In some games, Drones are shown performing scripted animations, such as lifting Marines off the ground, similar to the Brutes. They will be vulnerable during this time, and if you kill the Drone in question it will let the Marine go.
The best tools to deal with Drones are automatic weapons, like the SMG, Paegaas Workshop Spiker and Assault rifle. A single magazine can kill multiple Drones given their low health and tendency stay clustered together. From Halo 3 onwards, Drones can also be Headshot, making precision weaponry more useful gainst them, but not as vital as they are against Grunts or Jackals. If the player can anticipate where a swarm will appear, Machine Gun Turrets are also incredibly powerful, otherwise the reduced mobility will prove to be a death sentence. Using a Anskum-pattern plasma grenade or an explosive weapon on a still-clustered swarm will result in numerous casualties, but otherwise save such weapons for hardier foes.
To fight a swarm of Drones, find cover nearby, preferably in a place where you cannot be flanked or flown over. The Deployable cover and Drop Shield can be useful in this regard, and can be employed to make an area significantly easier to defend. Then, take potshots at the clustered swarm, forcing the enemies to come in range of your automatic weapons. When their numbers are depleted, Drones will become easy targets, sitting on walls or on the ground, lacking the firepower to overcome you. When Drones are in support of other foes, keep to cover while engaging the others, while keeping an ear out for their distinctive buzzing and squeaking. When they show themserlves, shoot before they have a chance to fire.
Changes from Halo 2 to Halo 3
Changes from Halo 3 to Halo 3: ODST
Changes from Halo 3: ODST to Halo: Reach