Here's what I've gotten from the books (X-Wing series and various stand alone SW books with good space combat.):
Particle shields: Necessary on all spacecraft, to defend against dust and other small debris in space. Military grade particle shields provide extra protection in order to defend against shrapnel, larger debris, and the occasionaly ballistic weaponry.
Energy Shields: Not strictly necessary, but they help defend against energy based attacks such as laser cannon bursts, ion cannon bursts, and EMP weapons.
From what I've read, particle shields tend to be limited to one shield generator that more or less covers the ship equally on all sides. However, they can/may be more like energy shields, which have emitters scattered over the surface of the ship. Energy shields' power can be redirected and changed by controlling the amount of energy poured into individual emitters.
I'm with Driver in the opinion that they cover ships like a skin, close over the surface...at least on most small craft. Larger ships, like SDs, have them further out. There are instances of starfighters flying between SDs and their shields, so at least Imperial craft work that way.
/geek-out
-Drac