There are many different reasons why your cat may become aggressive, and while, it may be noisy, dramatic and even upsetting (for both you and the cat) there is usually a way to solve the problem
The types of aggression a cat might display include;
Play aggression – this is usually seen in young kittens who are learning appropriate social behaviours. They learn to hunt by stalking their littermates, and if the play goes too far will “scold” one another. If a cat is not socialised appropriately at this age this play aggression will continue into adulthood
Fear/Defense aggression – this is an aggression that comes from the cat feeling endangered, vulnerable, or trapped.
Territorial aggression – this is perhaps the most common form of aggression between cats. Cats are very territorial and will protect jealously their domain
Petting aggression – this form of aggression is not well understood, although it is generally thought that it occurs because the cat has become over stimulated by the attention it is receiving. Different cats have different thresholds for affection so you should be aware of the cues in their body language.
Inter-Male aggression - this is aggression occurs largely between unneutered toms and is usually as a result of competing for territory, females and access to food.
Maternal aggression – this form of aggression is a protective response that is instinctual in mother cats - she may even become aggressive to cats she was formerly friendly with. This form usually subsides as her kittens are weaned.
Redirected aggression – this type of aggression is usually born from frustration or stress, which the cat cannot vent on to a particular target. For example, another cat outside the house can provoke aggression but because it is inaccessible, it is redirected toward another target, such as another resident cat or person.
Pain aggression – this aggression is a protective reaction as a result of the cat feeling pain, it may also occur in anticipation of pain too.
Idiopathic/ Unprovoked aggression – this is an aggression that is displayed which appears to have no observable cause. It is rare and can be incredibly dangerous for the animals and humans around a cat which is displaying this behaviour. It may have its routes in an underlying medical condition and so it is important to rule out medical causes and other forms of aggression first.
Reducing aggressive behaviour is easy, once you know what is causing it. After having ruled out any medical issues, you can work on eliminating or reducing the effect of the cause of the behaviour. Never use punishment as a deterrent. It is often the case that your cat will become more aggressive, particularly if it has its roots in fear.
How to manage aggressive cats
Cats are usually tolerant of other cats when sharing a house, however, there are always exceptions to the rule so before you add another cat to your home, consider your cat’s personality.
Make sure that you introduce cats to each other slowly, in a measure and organised way, and you should have cats that will be happy to share a territory.
Always make sure that your cats are neutered, intact cats – males particularly - tend to be more aggressive.
Make sure that you understand your cat’s body language and identify where their limits are. For new cat owners there is a wealth of information online and even courses you can attend but nothing can replace ”hands” on education. Learn about your cats and their behavioural cues and, gradually, as you become less fearful, so too will your cat and as a result less likely to strike out.
Experienced cat owners should also go back to basics. Perhaps something subtle has changed to make your pet act this way, and you just haven’t noticed, For example, your new perfume is upsetting her, or your children are playing a new computer game that sounds strange.
Redirecting a cat’s attention is a great way to help; some behaviourists suggest that interrupting a fight by clapping your hands or making a loud noise is effective, as is using a spray from a compressed air canister or water gun.
However, a less stressful way of diverting their attention you can try, if you see a fight brewing or a flight has started, is by redirecting their energy with a toy.

 
As well as using redirection, try to make an effort to build in play sessions to their day. An engaged and/ or exhausted cat is less likely to be aggressive! Not only will this redirect their energy it will also increase your bond, stimulate their mind and engage their “wild side.”
If the aggression is between cats in the same household, this could be due to a wide variety of reasons but can be managed in part by providing separate feeding bowls and litter boxes. This will reduce the idea that they are competing for resources.
You should also make sure that they have sufficient vertical territory as well as horizontal territory by providing cat trees or other spaces above ground level. This gives the cats more opportunity to spread out if they need some alone time.
Reward good behaviour, but make sure that you don’t treat too much otherwise your cats may put on too much weight and treats become food!
Always leave cats alone following a fight, otherwise they will redirect their anger upon you.
Finally, a great way to help reduce aggression in multi cat households is Feliway Friends, a synthetic copy of the cat appeasing pheromone (CAP) that is naturally produced by nursing felines. CAP helps cats feel safe and secure, and promotes a harmonious bond between them and their kittens

 
There are a number of other products on the market which also help reduce stress and calm your pets, including tablets, clothing and even toys. Great for reducing the anxiety related to travel, noisy events like thunderstorms, or fireworks, moving to a new home etc.
If you find, however, that your pet remains anxious do make sure you visit your vet to discover if there is any underlying medical issue or, perhaps, prescribe something to help reduce their symptoms.