How do magnetic shields work? I've come across the fact that magnetizable material can shield magnetic fields. For example, if I put a ferromagnetic plate between the magnet and a nail that is hanging on a thread pulled towards the magnet, the nail will eventually fall down when the plate covers the path between them. 
How exactly does this work? The descriptions I've found via google so far don't make sense to me. 
Probably there is a difference between static and dynamic fields, I'm mostly interested in the static case though.
 A: The intensity B of the magnetic field, given a fixed position, is proportionnal to the inverse of the permeability $\mu$ of the material. However, $\mu = \mu_0\mu_r$ where $\mu_0$ is the permeability of the void (it is a constant) and $\mu_r$ is the relative permeability of the material. Then,
$$ B \propto \frac{1}{\mu_r}$$
However, for air, $\mu_r \approx 1$, and for iron or ferrite (and many other materials) it can be more than 1000, which strongly diminish the intensity of magnetic field.
A: Perhaps you know that magnetic fields are closed loops. So if you influence such a magnetic field by another material the magnetic field lines take a different path. 

I've come across the fact that magnetizable material can shield magnetic fields.

Any material is magnetisable but in different strength. This has to do with the magnetic dipole moments of the subatomic particles of the material. If the particles could be aligned easily - like this is the case for ferromagnetic materials -the field lines are going along the shape of this material and in some regions outside the shape the magnetic field gets very weak or is not observable at all. By this

the nail will eventually fall down when the plate covers the path between them.

