The Feline Leukemia Virus (FeLV) was somewhat inappropriately named because it is different than the disease Feline Leukemia. FeLV is a virus that suppresses your cat's immune system. A cat that becomes infected with the virus becomes susceptible to many ailments or breakdowns in its system.
Cats that test positive for the virus will not necessary contract the disease Feline Leukemia. However, FeLV-positive cats are more likely to catch any one of a number of diseases including, but not limited to, Feline Leukemia, lymphoma or opportunistic infections.
FeLV is a virus that is specific to cats only. It CANNOT be transmitted to humans (including children), nor can it be transmitted to other species such as dogs. FeLV is transmitted via saliva, mucus, urine, feces and blood. This means that mutual grooming and biting/fighting are the most likely methods of transmission, yet sneezing, hissing, sharing food or water bowls and sharing litter boxes are also possible means of transmission.
There is no cure for this virus, but if you keep your FeLV-positive cat strictly indoors and in a healthy environment (i.e., away from any sick cats), there is no set time period for how long he/she will live. One person on the internet said they had a cat that lived for 20 years with the virus; others have given dates as long as 10 or 12 years. These, however, are probably extremes. The most important point to stress is that FeLV-positive cats MUST be kept away from other cats, although multiple cats with FeLV can live in the same household. Keeping your cat indoors - for their protection as well as the protection of other cats they would come in contact with - is one of the responsibilities of owning a FeLV-positive cat.