The Flood are a parasite. They have been a burden on the Halo Universe for much longer than just in the short time we've come to know it in the Halo Trilogy. The Flood are first encountered in the Halo trilogy in Halo: Combat Evolved, when Fire Team Charlie are exploring a swamp in search of a weapons cache on Installation 04. Unbeknown to them, they release the Flood from the last place they had been inhabiting on the Halo Ringworld. It is only a matter of time before the Infection Forms take hold of the marines and start spreading the infection. The Flood are a parasite that rely on another host to survive and once they do their outward appearance is similar to that of a zombie. They can resurrect fallen comrades and enemies alike and force their bodies into motion by invading the person's nervous system. There are multiple kinds of Flood that are encountered in all three of the games as well as the novels.
Many Halo different forms of the Flood have showed up during the course of the game.
 Infection Form
These little critters look harmless enough alone since they pop from a simple Assault Rifle burst, but don't let their looks fool you. These guys are known for their names. They are the main causes of all the other forms of the Flood. They infect and spread the infection by latching onto a living host and transfer their scourge directly into the bloodstream. It doesn't matter if the target in question is dead or alive, the Infection form will take control of the host's body and use it to carry out it's twisted will. Also in a group these buggers can swarm over Master Chief and quickly deplete his shielding.
These forms are, as their name implies, the main adversary that the Flood brings about. They come in alternate forms depending on who the Infection form took over. Infected Humans are usually thinner flood with smaller limbs and are easier blown apart. Infected Elites usually still carry their old armor, and are generally the most annoying Flood Combat form. The shields still sometimes function, making them harder to kill when in a big pinch. And the Brute Combat forms are usually bigger and more terrorizing than most of their counterparts. But they still all fall just the same.
These are one of the more annoying Flood forms. They are just as their name implies, carriers. They walk on stubby legs while the top half of their body is made of a bulbous and stretched skin-like material. Unfortunately, it acts the same as an infection form when a round is loaded into it, it explodes; but this is a bad thing, it releases a ton of Infections forms that just swarm and end up hurting you just the same as the explosion. The best thing to do to Carrier forms is to stick them with Plasma Grenades or Spike Grenades.While the Infection Forms are a major reason for the further spread of the infection, these forms are by far more dangerous then Infection forms. Because of their slow and blundering appearance, people tend to underestimate the Carrier form. This is the worst mistake you can make. The Carrier form is a veritable Infection Form bomb. Whe nit explodes, it can disorientate any people, infantry and Spartan alike, giving the Flood a chance that they will not think twice about taking. Infection forms can be latched onto infantrymen in the area before they've recovered from the shell shock of the Carrier Form, and even if they did manage to recover from the disorientation, they would have to deal with a wave of Infection form that will easily overcome most of the best trained soldiers.
These pure forms are a new addition to Halo 3 and are tougher than your run of the mill Flood. They are not actually one form, instead, they are one Flood that is able to turn into 3 separate types of enemies; and each of them packs a punch in their own way.
Little is known about how the Pure form actually come into being. They are a contradiction to the Flood as a whole when one thinks about it. The Flood forms are created when an Infection Form infects a human, but the Pure forms seem to come about from nothing at all. This would lead us to believe that, while the Flood are an infection, they also seem to have a way to grow and develop as beings even when a sentient being is not available to use as a host.
The Ranger form is exactly how it sounds, it's a ranged attacking Flood form. It attacks from a long distance, mounting itself on walls, grounds, ceilings, or just about everything. They're vicious bastards that shoot out sharp needles that tear into energy shields as well. They are also relatively tough to kill with most weaponry, but the most effective happens to be the Needler.
The Tank form is a pretty beefy and powerful form of Flood. It looks almost as if a Hunter was infected by the Infection forms. It's got a thick limbs and attacks with a charge similar to the Brute, which can end up sending you flying. The usual best result to take out these are assassinations or sticks. Your usual best bet is to run for cover until they happen to change shape again, since most of your hits do nothing to combat its thick hide. You can also hit their face with a melee attack. You can also shoot their face with any weapon like shotgun, energy sword etc.
Stalker forms are their quick and agile pure forms. They look similar to spiders and can sure run away. They have a great leaping ability which is used to attack, or they get far enough away to get themselves in a Ranger form and take you down. These are the weakest of the Pure forms, since they have really no defense and rely basically on their speed. Get 'em down fast and you're safe.
 General Flood control
When dealing with the Flood, it's important to remember that they can overrun you in the briefest of moments. Never underestimate them as an enemy. When faced with copious amounts of flood, it's important to know how to dispatch them properly. As a general rule, Covenant rules are not the best weapon for handling Flood. Human weapons that pack a punch, such as the Shotgun are preferable. If you have nothing except Covenant weapons at your disposal, try to use either the Plasma Rifle or Needler. Carbines and Needlers aren't very powerful against this foe and should only be used as a last resort against the Flood.
If you're only facing Infection Forms, any automatic weapon will handle them, such as the Assault Rifle or Plasma Rifle. It is only the larger Ranger, Combat or Tank forms that you need to think about upgrading your firepower to survive against. The Shotgun is best for taking down any Flood quickly, but when it comes to cutting a swathe through Floods of Flood, there is only one weapon to get the job done right.
When it comes to the best of the best, you should think of grenades and Flamethrowers as the best defense against the Flood. The Flamethrower is the cream of the crop. It can take out a room full of Flood in less than 20 seconds. This fire breathing dragon of a weapon incinerates anything in it's path, regardless of their number. This is the reason it is best against the Flood. Infection Forms or Tanks alike, all will be crisped by the Flamethrower.