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There are a variety of shields available in the realms, each with its
advantages and disadvantages. Each is outlined below, in order of increasing
size: 

Bucklers - These small, round shields are rarely much larger than the
wielder's head. Small and light, they can still provide some protection. They
provide a +1 bonus to one's armour class (AC) in all locations (which is
reflected in one's 'defence' score). They can not be used to 'bash' an
opponent. 

Light Shields - These relatively light, moderately sized shields are roughly
large enough to cover the wielder's chest. They provide a +1 bonus to one's
armour class (AC) in all locations (which is reflected in one's 'defence'
score), just as a buckler, however they can be used to 'bash' an opponent. 

Heavy Shields - These substantial shields are roughly large enough to cover
their wielder's entire torso. They provide a +2 bonus to one's armour class
(AC) in all locations (which is reflected in one's 'defence' score), and can
be used to 'bash' an opponent. 

Tower Shields - These massive shields are nearly the size of their wielder,
providing exceptional protection, but at substantial cost. They provide a +4
bonus to one's armour class (AC) in all locations (which is reflected in
one's 'defence' score), but impose a -2 penalty on hitrolls, apply a +2 limit
on dexterity bonuses, and can not be used to 'bash' an opponent. 

Using any shield incurs an armour penalty on certain skills and on attack
rolls. This penalty can be avoided for attack rolls by taking the shield
proficiency feat (or, for tower shields, the tower shield proficiency feat),
but the penalty still applies to skill checks.
 
* A shield's quality can affect its base AC value.