A good diet is really important for keeping your cat fit and healthy. Cats are obligate carnivores which means that they need a good amount of meat in their diet. It is not possible to feed a cat a vegetarian diet and expect them to remain healthy as a number of the nutrients they require can only be found in meat (e.g. taurine which is vital for a healthy heart).
The nutritional requirements of cats change as they age and depending on other health conditions. Diet is a vital part of treating diseases such as urinary tract problems and kidney failure.
Overfeeding will lead to obesity which can lead to diabetes, while underfeeding can also be detrimental. Allowing your cat free access to food all day is likely to lead to overfeeding, however feeding them in one large meal is very unnatural.
In the wild, cats will eat lots of small prey in a day so feeding lots of small meals is ideal and will help reduce boredom.
Providing plenty of fresh water is also important, although cats are not always very good at drinking sufficient. A lot of cats prefer filtered and/or running water and special cat water fountains can be bought to cater for this need.