My cousin has it and I was wondering what exactly it is?
In haemophilia the blood is missing a vital clotting factor, so it doesn't clot properly. This means that any sort of cut or scratch or wound will keep bleeding for much longer than in normal people, so they can lose a lot of blood from an injury that in other people is pretty trivial. They are often given regular transfusions of clotting factor, though this of course carries a risk itself of introducing viruses from contaminated blood.
It's genetic, caused by a bad gene on the X chromosome. Women have two X chromosomes, and both have to be bad for them to have the disease, which is very unlikely. If they have one bad gene and one good gene they are carriers, but don't have the disease. Males have only one X chromosome, inherited from their mother, and if they get a bad one, they get the disease. It is possible for the bad gene to form by mutation in an egg cell, so it can suddenly appear in families.
For what it's worth, haemophiliacs are not likely to get strokes or other thromboses.
There are some really good coagulant drugs on the market if he injures himself to promote clotting.