There are two types of shields, square shields and kiteshields. Square shields have a good defence against stabbing and slashing attacks but have a low defence against crushing attacks. The kiteshield has a higher defence against slashing and crushing attacks, but a lower defence against stabbing attacks. Note that you can't have a shield equipped at the same time as a two-handed weapon, like the halberd. Magical staves are not considered two-handed and can be used with shields or other off-hand weaponry.
A White sq shield has the same stats as a Black sq shield, but also has a +1 Prayer bonus. It requires level 10 attack, 10 prayer and completion of Wanted!.