[Merlin]

Ceramic ones are usually sand-filled. This prevents the wire vapourising onto the outside of the tube and continuoing to conduct after blowing- this is pretty much overkill for things like radios.

In theory, a quick blow fuse will blow in a matter of milliseconds if current exceeds the ratied value. A time delay or slow-blow fuse won't, so that if the current is only a brief transient the circuit will keep on working, it will only blow if the surge is sustained for more than a couple of seconds. This is handy for things like inrush current where you don't actually want the thing to blow. You'd probably use time delay type fuses in the HT line.
In practice I find there to be little difference between quick and slow blow types, they seem to be much of a muchness, at least when working with radios/amplifiers. I guess it makes a difference with more sensitive circuits. Maybe others disagree?..