Electrons have intrinsic angular momentum
characterized by quantum number
1/2. Electron spin is a figurative
label for this property of electrons, with no fundamental analogy
in our everyday lives. Every electron
exists in one of two states, either spin-up or spin-down. This "spin" of electrons produces what's called a magnetic moment
A ball of charge can have a magnetic moment if it is spinning such that the charge at the edges produced an effective current loop. This idea led to the use of "electron spin" to describe the intrinsic angular momentum of an electron.
To put it simply, if a significant majority of the electrons in a metal are in either spin-up or spin-down state, the metal will be magnetic.
Electron spin is the spotlight principle in a new field of technological development known as spintronics.
Someone has to be able to describe this better than me :)