Peter Munro has written an excellent introduction to the subject. His mention of the rather specialised FF fuses reminds me of the definition of a semiconductor:
A device that blows in order to protect the fuse
If you have to replace a blown glass fuse it's worth a careful look at the old one. If it's badly blackened or even shattered then there was a short circuit. If most of the wire is intact with no evidence of violence it may have just died of old age or possibly a mild long term overload. It's pretty obvious that you would use different methods to diagnose these different faults.