There are four shield types in Second Edition - body, buckler, medium, and small. The benefits and penalties are clearly spelled out in the rules.
Shields: All shields improve a character's Armor Class by 1 or more
against a specified number of attacks. A shield is useful only to
protect the front and flanks of the user. Attacks from the rear or rear
flanks cannot be blocked by a shield (exception: a shield slung across
the back does help defend against rear attacks). The reference to the
size of the shield is relative to the size of the character. Thus, a
human's small shield would have all the effects of a medium shield
when used by a gnome.
A buckler (or target) is a very small shield that fastens on the
forearm. It can be worn by crossbowmen and archers with no hindrance.
Its small size enables it to protect against only one attack per melee
round (of the user's choice), improving the character's Armor Class by
1 against that attack.
A small shield is carried on the forearm and
gripped with the hand. Its light weight permits the user to carry
other items in that hand (although he cannot use weapons). It can be
used to protect against two frontal attacks of the user's choice.
medium shield is carried in the same manner as the small shield. Its
weight prevents the character from using his shield hand for other
purposes. With a medium shield, a character can protect against any
frontal or flank attacks.
The body shield is a massive shield reaching
nearly from chin to toe. It must be firmly fastened to the forearm and
the shield hand must grip it at all times. It provides a great deal of
protection, improving the Armor Class of the character by 1 against
melee attacks and by 2 against missile attacks, for attacks from the
front or front flank sides. It is very heavy; the DM may wish to use
the optional encumbrance system if he allows this shield.