Benefits
When you use a shield (except a tower shield), the shield’s armor check penalty only applies to Strength– and Dexterity-based skills.
Normal
When you are using a shield with which you are not proficient, you take the shield’s armor check penalty on attack rolls and on all skill checks that involve moving.
Special
Barbarians, bards, clerics, druids, fighters, paladins, and rangers all automatically have Shield Proficiency as a bonus feat. They need not select it.