Shield, Buckler is a form of small shield that protects the user against but one opponent (of the user's choice) during any melee round in which the shield is employed.

It is small and easiy carried, and may be strapped to the arm, making it able to be used by crossbowmen and slingers.
A small demi-human such as a halfling or a gnome would be fully protected by the buckler, and it is treated as a small shield in those cases.

BULK: non-
WEIGHT*: 3#
BASE MOVEMENT: -
COST: 5 gp

* Assumes human-size.

Shield USE:
A shield is basically a barrier between its wielder and his or her opponent.
It is used to catch blows or missiles.
It can also be used offensively to strike or push an opponent. <Shield Bash, Shield Rush>
The shield can be used fully only to the left or front of the right handed individual.
Attacks from the right flank or rear negate the benefits of a shield.

Magic Shields:
Magic shields are no less weighty than their non-magical counterparts, but they are non-bulky with respect to encumbrance.
 
 
Descriptions of New Armor Types TYPES OF ARMOR & ENCUMBRANCE - DMG Unearthed Arcana